Russian President Vladimir Putin has arrived on a state visit to the UK - the first by a Russian leader since 1874.
Putin joined the Queen in a horse-drawn carriage
Correspondents say the visit is a reflection of the rapid development of ties between Britain and Russia since the end of the Communist era.
There will be a particular focus on co-operation in the energy sector, amid the pomp and ceremony that tradition demands.
Mr Putin was greeted at Heathrow airport by the Prince of Wales, before being driven to central London to meet the Queen and UK Prime Minister Tony Blair.
Tuesday: Official welcome; Buckingham Palace banquet
Wednesday: Visit to Edinburgh; banquet in London's Guildhall
Thursday: Russia-UK energy seminar; Downing Street lunch
Mr Putin joined the Queen in an open-topped horse-drawn carriage for the final part of the journey to Buckingham Palace.
On the eve of the visit, Mr Blair said new oil and gas deals signed by UK companies would soon make the country Russia's biggest foreign investor.
BBC diplomatic correspondent Mike Wooldridge
says British officials seem confident that the very public differences between Mr Blair and Mr Putin over the Iraq war will not affect the atmosphere of the state visit.
Russian officials also say the two countries' views of how to handle Iraq have been converging.
When Tony Blair and Mr Putin hold talks on Thursday, the Middle East peace process is also likely to be on the agenda.
International concern over Russian support for Iran's nuclear programme may also feature, and Mr Blair has pledged to raise anxieties regarding the conflict in Chechnya.
Mr Blair and Mr Putin will both attend the opening of a Russia-UK energy conference on Thursday, which marks the agreement of some major UK investments by BP and Shell in the Russian energy sector.
The BP deal is worth $6.75bn and will set up an oil company, TNK-BP, that will be Russia's third largest.
Shell is part of a consortium investing $10bn in the oil industry in Sakhalin, in the Russian Far East.
Mr Putin's arrival on time at Heathrow airport is in itself an improvement on the last state visit by a Russian leader, when the yacht carrying Tsar Alexander II and his party ran aground off the Dutch coast.