David Cameron has welcomed Indian Prime Minister Narendra Modi to Downing Street, pledging to deepen ties between the countries.
The leaders shook hands on the steps of Number 10 before going inside for talks with their ministers around the Cabinet table.
Mr Cameron told his counterpart - the first Indian premier to visit the UK for a decade - that "relations between our peoples are already very strong" and he hoped that other links could be improved.
Rather than pulling into Downing Street in his convoy, Mr Modi was brought in on foot via the Foreign Office, apparently to avoid noisy protests taking place outside.
After initial talks the leaders will hold a press conference, and meet again later at the PM's Chequers residence.
Mr Modi is also due to meet the Queen at Buckingham Palace during his visit.
Britain is already the largest investor in India among G20 countrie while India invests more in the UK than it does in the rest of the EU combined.
Billions of pounds worth of commercial deals are expected to to be signed over the coming days - including plans for the UK to become a centre of offshore rupee bonds and a partnership to develop three "smart cities" in India.
Mr Cameron has branded the visit an "historic opportunity" for two nations "tied by history, people and values, to work together to overcome the biggest challenges of our age".
Mr Modi has said the aim is "strengthening cooperation with a traditional friend". The premier's itinerary also includes an address to the British Parliament - despite the fact Parliament is not in session - and an event at Wembley Stadium.
But while he is receiving a rapturous welcome in some quarters, Mr Modi - who won the world's biggest democratic mandate in elections last year - is also coming under fire from critics.
A crowd of around a hundred protesters had gathered outside Downing Street, chanting noisily and holding placards.
Messages on the banners included "Modi not welcome", "Stop religious persecution", and "Remove illegal blockade in Nepal".
A police cordon and vans were in position to prevent them disrupting access to the street. Roads around Westminster were closed off and helicopters were circling overhead.
More than 200 writers, including Ian McEwan, Salman Rushdie and Val McDermid, have written an open letter to the Prime Minister urging him to raise concerns about freedom of expression in India during his talks with Mr Modi.