The King quoted Shakespeare to describe his late mother as a "pattern to all princes living" in a moving inaugural address to both Houses of Parliament.
Both speakers of the Houses of Parliament paid tribute to Queen Elizabeth II in a ceremony dedicated to the late monarch at the Palace of Westminster, as the new sovereign and his wife, Camilla, watched on.
Parliament, though often a place of disagreement, gathered on Monday morning to express their condolences in one of the most important ceremonies during the nation's period of mourning.
King Charles III, dressed in black, thanked them for their tributes that "so touchingly encompass" what "our late sovereign, my beloved mother, the Queen, meant to us all".
In an address to around 900 Lords and MPs, the King quoted William Shakespeare as he described his mother’s legacy and spoke of feeling the “weight of history” as he stood inside the historic room.
Speaking from a gilded lectern, he said: “As Shakespeare says of the earlier Queen Elizabeth, she was ‘a pattern to all princes living’.”
He continued: "As I stand before you today, I cannot help but feel the weight of history which surrounds us and which reminds us of the vital Parliamentary traditions to which members of both Houses dedicate yourselves with such personal commitment, for the betterment of us all.”
The King vowed to "faithfully follow" his mother's "example of selfless duty" in his new role, as he described Parliament as the “living and breathing instrument of our democracy”.
“While very young, Her late Majesty pledged herself to serve her country and her people and to maintain the precious principles of constitutional government which lie at the heart of our nation," he said.
Watch ITV News' special coverage of the King's first speech to Parliament
“This vow she kept with unsurpassed devotion. She set an example of selfless duty which, with God’s help and your counsels, I am resolved faithfully to follow.”
Police flanked the road in Parliament Square, which was shut down to traffic, while crowds and reporters lined the pavements ahead of the new monarch's arrival.
People erupted into cheers with many raising their phones to take photos and videos as the royals' motorcade arrived and drove into the gates, with Charles and Camilla waving at the waiting crowds.
Prime Minister Liz Truss and leader of the opposition Sir Keir Starmer were among those sat together in Westminster Hall with other senior figures from the Commons to one side of the throne, placed at the head of the hall.
MPs will not sit in Parliament until after the Queen's funeral on Monday, September 19.
Lord Speaker Lord McFall of Alcluith and Speaker of the Commons, Sir Lindsay Hoyle, led messages of condolences to the Queen.
Westminster Hall has been at the heart of the way the country has been run since it was built almost 1,000 years ago, and it is where the Queen will lie in state from Wednesday.
First, the Queen will be taken later today from the Palace of Holyroodhouse to nearby St Giles’ Cathedral where her family, and a congregation drawn from all areas of Scottish society, will attend a service of thanksgiving for her life.
Charles will lead some of the royals – expected to be the Duke of York, Earl of Wessex and the Princess Royal and her husband Vice Admiral Sir Tim Laurence – on foot, while the Queen Consort and other members of the monarchy follow in cars.
Members of the public will be able to view the coffin to pay their respects for 24 hours before it is taken to London.
People have been warned to expect 20-hour waits to see Her Majesty's coffin, as a quarter of a million people are expected to descend on London this week, with the Met Police's new commissioner calling preparations a “massive challenge” for the force.
During the ceremony at the Palace of Westminster, the Speaker of the Commons extended the sympathy of MPs to the new monarch, as well as pledging loyalty to the new King.
“In 1988, we celebrated the 300th anniversary of the revolutions of 1688 to 1689,” he told the Hall.
“It is perhaps very British to celebrate revolutions by presenting an address to Her Majesty; but those revolutions led to our constitutional freedoms, set out the foundation for a stable monarchy, which protects liberty.”
The Lord Speaker also pledged his loyalty to Charles, as he praised the Queen’s “inspiring reign of deep and unparalleled devotion”.
Lord McFall read an address, unanimously agreed by peers, conveying “the deep sympathy felt by this House in the grief Your Majesty has sustained".
“To assure Your Majesty that the example of selfless public service, which our late sovereign displayed over her reign over 70 years, her untiring endeavours for the welfare of her peoples, and her fortitude in adversity will ever be held in reverent, affectionate and grateful remembrance," he told the King.
“And to express to Your Majesty our loyalty to Your Majesty’s royal person and our firm conviction that under the blessing of divine providence Your Majesty will throughout your reign further the happiness and protect the liberties of all your peoples in all your realms.”
The ceremony ended with both Charles and the Queen Consort standing as the national anthem was played.
Later on Monday evening, the King and other members of his family, likely his siblings, will hold a vigil at St Giles' Cathedral in honour of the Queen.
The Duke of Sussex earlier released an emotional statement, promising to “honour” his father as the new King.
His comments came after the King said he wished to express his "love" for his son and his daughter-in-law Meghan Markle "as they continue to build their lives overseas" in his inaugural speech to the nation on Friday.
The remarkable life of the Queen remembered in our latest episode of What You Need To Know
The King's relationship with his youngest son had reportedly been fraught since he and wife Meghan decided to step down as working royals and move abroad - but many the King's words of affection during his speech as an acknowledgement they have put their differences to one side.
Harry’s emotional statement also paid tribute to his grandmother’s “everlasting legacy”, saying: “You are already sorely missed, not just by us, but by the world over.”
He reflected on his “first meetings” with the Queen, including “the first moment you met my darling wife and hugged your beloved great-grandchildren”.
And in a poignant final line referencing the late Duke of Edinburgh, he said: “We, too, smile knowing that you and grandpa are reunited now, and both together in peace.”