Prince William, Boris Johnson and chief medical officer Chris Whitty were among the guests who joined NHS staff and volunteers at a St Paul's Cathedral ceremony to honour the health service's 73rd birthday.
The hour-long service, which began from around 11am, celebrated the contribution of all NHS staff and volunteers during the pandemic.
Other guests at the socially distanced ceremony include health secretary Sajid Javid, Labour leader Sir Keir Starmer, deputy chief medical officer Jonathan Van-Tam, NHS England national medical director Prof Stephen Powis and NHS chief executive Sir Simon Stevens.
The Duchess of Cambridge was also due to attend with her husband but had to withdraw after coming into contact with someone who tested positive for Covid-19.
The Queen has also awarded the George Cross to the NHS.
In a personal, handwritten message, the Queen said NHS staff have carried out their work “with courage, compassion and dedication” for more than 70 years.
The George Cross - which is made on the advice of the George Cross Committee and the Prime Minister - is granted in recognition of “acts of the greatest heroism or of the most courage in circumstances of extreme danger”.
Ahead of the service, party leaders spoke for a few minutes before taking their seats at the front of the cathedral as organ music played.
The Duke of Cambridge arrived afterwards and as a choir clad in white robes sang hymns, he followed religious leaders past rows of NHS staff.
Addressing the congregation, NHS chief executive Sir Simon described the occasion as an opportunity for “cautious pride in science, treatments and our vaccines” but also a time for some “anger and regret” over the millions who have died with coronavirus around the world.
He described the NHS as an example of “building back better” and “an inspiring example for our generation of how out of adversity can come strength”.
Later, Prince William will host a garden tea party for NHS staff at Buckingham Palace.
He will meet staff, including respiratory ward nurses, counsellors, care workers, and those in non-clinical roles such as catering managers and housekeeping staff.
The event hosted organised by NHS Charities Together is one of thousands of Big Teas taking place across the UK on Monday.
Alexandra Heys, nurse ward manager in the respiratory high care unit at Gloucestershire Hospitals NHS Foundation Trust, said the tea at Buckingham Palace "means a lot to NHS staff who have been through so much over the last year" and said she feels honoured to be meeting the Duke of Cambridge on Monday.
She continued: “I know from personal experience that the past year has taken a huge toll on NHS staff who have worked incredibly hard to take care of patients during a challenging time.
“My team at the respiratory high care unit has cared for over 300 very sick Covid patients, all of whom required dedicated around-the-clock care.
“All NHS staff have gone above and beyond, providing additional care in difficult circumstances, including connecting isolated and very sick patients with their families in their most challenging moments."
Sam Foster, who administered the first out-of-trial Oxford/AstraZeneca vaccine says having Prince William join the service makes it 'extra special'
Sam Foster, chief nursing officer Oxford University Hospitals, who administered the first Oxford/AstraZeneca vaccine to a patient outside of clinical trials on January 4 said Monday was a time to stop and reflect on the last year and a half of the pandemic.
She said: "It's a real honour. I'm just really delighted. I've never been inside St Paul's Cathedral. So to be invited to represent my profession and my organisation is a real honour."
She added: "I only just heard on the news this morning that members of the Royal Family are going to be joining the service. That just really makes it extra special."
The nurse said NHS staff wellbeing is the "absolute priority at the moment", saying staff are "very tired" and some are "quite damaged by the last year".
Speaking about giving the first out-of-trial AstraZeneca jab, she said: "It probably wasn't till afterwards that I just really stopped and thought what a historical moment it was. If you remember, it was just after new year we were still in the peak of the pandemic. We were all working very very hard."
Dr Ashley Price, who treated the first coronavirus patients in the UK, spoke about the 'real horrible times'
Dr Ashley Price, infectious disease consultant in Newcastle, who treated the first coronavirus patients in the UK, said: "I'm deeply honoured to be here today. This is about celebrating the NHS and its 73-year history and I think over the last year, the NHS has shown what it can do.
"I think it's also a time to reflect on relatives and family memers that we've lost and people who've been ill with coronavirus over the last year. So it's a commemoration but also a moment of reflection as well."
He reflected on the toughest times during the pandemic: "There have been some huge emotions over time, from real horrible times when I remember speaking to the first families who had relatives in hospital who were critically unwell. It was very very hard."
But he also pointed out there were moments of elation when the NHS gave the first Covid vaccinations.
Speaking about his role in treating the first coronavirus patients in the UK, Dr Price said: "All of those patients who were admitted in the early times were actually very well and they were all discharged well.
"And I think that that didn't reflect what was going to happen. Within a few months, we saw the consequences of this really dangerous virus and it hit the hospitals with many people dying and it was hugely hugely difficult time."
Dr Price also welcomed the planned reopening on July 19, but said he thinks face masks will still be needed in some settings.
Speaking at St Paul’s Cathedral, where he was due to address guests, he said: “The news is looking good.
“We’re having fewer admissions to hospital and those that are coming in we’re generally able to treat.
“We do have to remember though that there are a lot of vulnerable people in our society – people for whom the vaccine may not have worked as well – and we need to be mindful of that.
“I think, in places like hospitals, mask-wearing will need to continue for a bit longer because we are still contracting coronavirus and it’s possible to spread it to very vulnerable patients.”
NHS England national medical director Prof Stephen Powis said the best birthday present for the NHS is for those who haven't had the jab yet to get one soon
NHS England national medical director Prof Stephen Powis said the best birthday present for the NHS is for those who haven't had the jab yet to get one this week.
He said that with vaccines over 90% effective at preventing hospitalisation from Covid-19, the government "can look, as it is doing, at relaxing those social distancing measures, also looking at the data, and allowing the vaccines to do the work".
Speaking about the hard work of NHS staff, he said: "Everybody working in the NHS at the frontline - whether you're a doctor, a nurse, a porter, a paramedic, a receptionist - everybody has had to pull together in, I'm sure, what's been the greatest challenge of their working careers."
Commenting on pay, he said: "Look, I'm the national medical director of the NHS, you wouldn't expect me to say anything other than I want staff to be rewarded as much as possible.
"Of course, pay is an important component of that as is the show of public support that you've seen today here at St Paul's and of course through the award of the George Cross.
"There are independent review bodies that will advise the government on pay. Let's see what they say."
In a personal message, the Queen wrote: “It is with great pleasure, on behalf of a grateful nation, that I award the George Cross to the National Health Services of the United Kingdom.
“This award recognises all NHS staff, past and present, across all disciplines and all four nations.
“Over more than seven decades, and especially in recent times, you have supported the people of our country with courage, compassion and dedication, demonstrating the highest standards of public service.
“You have our enduring thanks and heartfelt appreciation.”
Details of the presentation of the award will be confirmed at a later date.
England footballer Jordan Henderson, who helped put his team through to the Euro 2020 semi-finals, said NHS staff are the “real heroes”.
Henderson, who is an Official NHS Charities Together Champion, said: “After the last 16 months, this nation owes so much to the NHS and the least we can do this July is encourage everybody to express their collective gratitude for that selfless support."
NHS Charities Together is a charity and umbrella organisation made up of 240 NHS member charities based within hospitals, mental health trusts, ambulance trusts, community health trusts and health boards.
William and Kate became its joint patrons in December 2020.