India's Prime Minister Narendra Modi is due to arrive in the UK later for a three-day visit as a guest of his British counterpart, David Cameron.
The first day of the visit will be marked by a Red Arrows flypast before Modi makes a speech to Parliament.
A highlight is expected to be Modi's address to huge crowds at London's Wembley Stadium on Friday night.
Cameron said the visit marked "a historic opportunity" for Britain and India to help each other prosper.
"It's an opportunity for two countries, tied by history, people and values, to work together to overcome the biggest challenges of our age," Cameron said.
"Prime Minister Modi and I intend to grab that opportunity with both hands."
Modi said the aim of the visit was to strengthen "co-operation with a traditional friend".
Downing Street protests
The visit comes at an unsettled time in India, where Modi's Hindu-nationalist party lost a recent regional election.
The Bharatiya Janata Party (BJP) suffered the defeat in the northern Indian state of Bihar, amid concerns over a rise in religious intolerance in India.
His supporters will hope his visit will put him on the global stage and help him spring back from that defeat.
But protests are planned outside Downing Street with opponents dressing all in white, the colour of mourning in India.
Campaigners from the UK-based Awaaz Network say they are against Modi's "violent authoritarian agenda that seeks to undermine India's democratic and secular fabric".
Modi's arrival in London will be marked with a flypast by the RAF's aerobatic team, the Red Arrows, over the House of Commons.
During the visit, Modi will stay at Chequers, Cameron's official country retreat in Buckinghamshire.
On Friday, Modi will speak, mainly in Hindi, to some 60,000 people expected at the Wembley event, which is expected to be a celebration of the Indian diaspora's contribution to the British economy.
Organisers have promised an Olympic-style reception for the Indian prime minister.
He will also visit the statue of Indian statesman Mahatma Gandhi, which stands alongside those of British prime ministers Benjamin Disraeli and Sir Winston Churchill in London's Parliament Square.