The Queen was greeted by the Rev John Hall outside London's Westminster Abbey before Tuesday's service marking the 200th anniversary of the abolition of the slave trade.
Demonstrators protested against the service and called for reparations to be paid to slavery's victims
The Queen laid a wreath at the monument to William Wilberforce in the Abbey
Protester Toyin Agbetu, 39, halted proceedings by shouting: "You should be ashamed. This is an insult to us."
Mr Agbetu walked from the congregation in the south transept to a space close to where the Queen and the Duke of Edinburgh were seated.
The prime minister and his deputy John Prescott with their wives and Owen Arthur, the PM of Barbados