India's most famous landmark, the Taj Mahal, has been closed to tourists following violence in Agra.
An angry mob clashed with police, after a crowd "went berserk after a speeding truck killed four people", state police chief Vikram Singh told the Associated Press news agency.
At least 50 policemen were injured and one man killed as trucks were set on fire in the Naj Mandi area of the city.
Backpackers looking to visit the iconic monument have been advised to remain in their hostels, with a curfew imposed in six areas of Agra.
The road accident has also sparked religious unrest, with Muslim demonstrators attacking neighboring Hindus.
The four men killed were all Muslims and had been returning from the Shab-e Barat festival.
Shab-e Barat (or the night of forgiveness and atonement) is a Muslim festival of mourning, during which families visit graveyards to pray for the souls of dead relatives.
JN Chamber, a spokesman for the state government, told the Times: "The cause of this violence was that traffic was not regulated properly. Trucks were allowed into a no-entry zone meant for pedestrians who were going for the Shab-e-Barat procession."
