US President George W Bush has arrived in India after a surprise stop-over in Afghanistan on day one of his first visit to South Asia.
Some 100,000 people rallied in Delhi against the Bush visit
Ahead of his arrival, tens of thousands of people protested in India.
Huge protest rallies were held in Delhi and Calcutta. Speakers said he was not welcome and condemned the Iraq war.
Top officials from both countries have been working to finalise a landmark nuclear deal which could be the cornerstone of Mr Bush's visit.
The deal gives India access to US civilian nuclear energy.
Heavy security was in place as President Bush landed in Delhi. The streets surrounding his hotel were sealed off as hundreds of policemen and US secret service agents took up position.
In a rare gesture, Indian Prime Minister Manmohan Singh personally received Mr Bush and his wife, Laura, at Delhi airport.
'Bush go home'
Earlier, about 100,000 protesters gathered in the heart of Delhi in the first of many planned demonstrations against the US president.
Most of them were Muslim. They held up black flags and placards reading: "Bush, go home."
Heavy security is in place outside Mr Bush's hotel
"We are against Bush because he is the enemy of Islam," one protester, Shamsuddin Malik, told the BBC.
"He is a war criminal. America has invaded Iraq and Afghanistan. Their policy towards humanity is deplorable," said Mohammad Anwar Hussain of the Jamiat Ulama-I-Hind, a hardline Islamic group.
The BBC's Sanjeev Srivastava in Delhi says that although many are protesting against the Bush visit, there are also many Indians who will welcome him.
Other protests took place in the eastern city of Calcutta and Bangalore and Hyderabad in the south.
In Calcutta, tens of thousands of supporters of left-wing parties marched through the city centre.
Many told President Bush to go home and stop the killings in Iraq. Some called on India not to become what they called a US lackey.
Indian and US officials have been trying to finalise a landmark deal which will give India much-needed access to US civilian nuclear technology.
"Our people are talking to the Indians, today on the plane," President Bush said during his stop in Kabul.
"Hopefully we can reach an agreement, and if not we will continue to work on it until we do," he said.
The deal was agreed to in principle during a visit by Indian Prime Minister Manmohan Singh to Washington last year.
But it has been held up by differences over plans to separate India's civilian and military nuclear programmes and open its civilian nuclear facilities to international inspectors.
During his trip, President Bush will visit the southern city of Hyderabad, one of India's high-technology hubs.
Mr Bush will head to Pakistan after India.
In a BBC interview broadcast on Wednesday, Pakistani President Pervez Musharraf said he hoped the US leader would be able to play a role in resolving the long-running Kashmir dispute with India.