Some 180,000 protesters have marched through central London in solidarity with the Palestinian people, organisers estimate.
Other demonstrations took place across the UK, including in Belfast, Cardiff, Plymouth and Manchester.
Police said the protest in London was largely peaceful and that seven arrests were made.
It was the worst violence since 2014 and saw Hamas fire a barrage of rockets into Israeli towns, while Israeli airstrikes targeted Hamas’ infrastructure - including a vast tunnel network - and skirmishes broke out between residents of Israeli towns.The Gaza Health Ministry says at least 243 Palestinians were killed, including 66 children, with 1,910 people wounded.
It does not differentiate between fighters and civilians.
Twelve people were killed in Israel, all but one of them civilians, including a 5-year-old boy and 16-year-old girl.
In the capital, demonstrators congregated at Victoria Embankment on Saturday afternoon before making their way to Hyde Park.
Holding banners, placards and flags, they let off green and red-coloured smoke and chanted, blocking traffic as they marched.
Some demonstrators could be seen wearing costumes, masks and face paint, while others were draped in the Palestinian flag.
Groups of police watched as some protesters climbed bus stops and lampposts and wrote “Free Palestine” on walls.
Chants of “Israel is a terrorist state” and “We are all Palestinians” could be heard during the march.
As the crowd reached Downing Street, chants of “Boris Johnson, shame on you” rang out.
A temporary stage was set up in Hyde Park where a number of speeches, including from former Labour leader Jeremy Corbyn, were made during the afternoon.
Mr Corbyn spoke at the end of the event, after speeches from former shadow chancellor John McDonnell and rapper Lowkey.
“Children should not grow up having seen their home demolished by a bomb dropped from the safety of thousands of feet above," the Labour MP said.
“The occupation is the issue, the occupation of the West Bank, the siege of Gaza, they are the issues.
“While the ceasefire is a step forward, unless the fundamental issues are addressed, we will be here again.”
Mr Corbyn added: “We will be here as long as is it takes until the Palestinian people are free.
“We will never give up on the Palestinian people, we will never go away from the Palestinian people and their cause.”
He was welcomed to the stage to large applause and chants of “Oh Jeremy Corbyn”.
Mr McDonnell said: “Yes, a ceasefire has been negotiated and we welcome a ceasefire.
“But let’s be clear, there will be no ceasefire in our campaign to boycott, disinvest and sanction the Israeli apartheid state.
“The message is clear, we will not cease our campaign in solidarity until there is justice.
“So let’s make it clear, no justice, no peace.”
Muktha Ali, 32, from Harrow, north-west London, said she was protesting "because this is now urgent, the Israeli occupation needs to end now, it’s been long enough.
“Mothers, fathers, brothers, sisters and children have been bombed and murdered, Palestine has to be free.”
Another, named Jake, said: “I stand in solidarity with Palestinians, and so should everyone, no matter your nationality.
“It’s great to see so many people come out in their thousands to support the cause. We will win – our cause will be heard.”
While largely peaceful, a few people could be seen throwing objects at police who attempted to split the crowds in Hyde Park.
Police said seven men were arrested, one on suspicion of a racially aggravated public order offence, another on suspicion of a homophobic public order offence, two on suspicion of going equipped to steal and three on suspicion of violent disorder.
Saturday marked the first full day of a truce that ended the fourth Israel-Hamas war in just over a decade.
Israel’s security cabinet has approved a ceasefire, a move praised by Boris Johnson and Foreign Secretary Dominic Raab.
ITV News Senior International Correspondent John Irvine reports from Gaza, as Palestinians returned to the city on Friday:
On Friday, Israeli Prime Minister Benjamin Netanyahu warned against further attacks, saying: "If Hamas thinks we will tolerate a drizzle of rockets, it is wrong."
He vowed to respond with “a new level of force” against aggression anywhere in Israel.
He added that Israeli forces had caused “maximum damage to Hamas with a minimum of casualties in Israel.”