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The King and Camilla, the Queen Consort, sat on thrones in Westminster Hall as they listened to condolences from the Lord Speaker and the Speaker of the House of Commons.
The King pledged to follow his late mother's example of "selfless duty".
Scotland farewells the Queen
Hundreds of lawmakers crowded into the 1000-year-old Westminster Hall at the Houses of Parliament for the service, rich in pageantry, in which Parliament offered its condolences to the King, and he replied.
A trumpet fanfare greeted the king and his wife Camilla, the Queen Consort, as they entered the hall, which was packed with hundreds of legislators.
The King told members of the House of Commons and House of Lords that he would follow his late mother Queen Elizabeth II in upholding "the precious principles of constitutional governance" that underpin the UK's political system.
The ceremony was held in Westminster Hall because monarchs are not allowed inside the House of Commons.
That rule dates from the 17th century, when King Charles I tried to enter and arrest lawmakers.
That confrontation between crown and Parliament led to a civil war which ended with the king being beheaded in 1649.
The queen's coffin will lie at the cathedral for 24 hours, giving members of the public a chance to file past and pay their respects.
On Tuesday, it will be flown to London where the coffin will lie in state at the Houses of Parliament Palace from Wednesday afternoon until the morning of the funeral on September 19.
Authorities already have issued rules and guidelines for people wanting to pay their respects in London, with a long queue expected.
Tomorrow, the King and Camilla will make a trip to Belfast, Northern Ireland.
The couple will visit Hillsborough Castle and view an exhibition on the Queen's long association with Northern Ireland.
The King will then meet the secretary of state for Northern Ireland in addition to other party leaders, and receive a message of condolence led by the speaker of the Northern Ireland Assembly.
Back in Scotland, the Queen's only daughter Princess Anne will prepare to accompany her mother's body as it is flown back to London.
The Queen's life in pictures