Police have shot dead 18 civilians in the deadliest day in Indian-administered Kashmir since protests erupted three months ago.
A policeman was also killed when he was run over by a lorry.
The BBC's Altaf Hussain in Srinagar says reports of Koran desecration in the US have stoked anger.
Scores of Kashmiris have now died since June, when anti-India protests broke out after police shot dead a teenager.
In Monday's protests, thousands of people defied curfews and took to the streets, chanting anti-India and anti-US slogans and burning effigies of US President Barack Obama, our correspondent says.
An angry mob set fire to several government buildings and a Protestant-run school, as well as attacking a police station, he adds.
Police fired live ammunition to break up the demonstrations, and confirmed that 18 civilians had been killed.
Several of the deaths were reported to have occurred in Budgam district, with others reported in the village of Tangmarg, where the school was burned.
One of those killed was a student aged 12 or 13, our correspondent says.
More than 100 people are also reported to have been wounded, some seriously, he adds.
Meanwhile, a policeman died after he was run over by a lorry driven by demonstrators in the town of Humahama.
The attack on the missionary school was condemned by separatist leader Syed Ali Shah Geelani - who has been put under house arrest by Indian authorities.
"I urge the Muslims to protect members of [the] minority community and their religious places. We should at any cost maintain the age-old communal harmony and brotherhood for which Kashmir is known the world over," he said.
A curfew was also imposed in the town of Poonch in Indian-administered Kashmir on Monday following violent protests, Indian officials said.
Protesters stoned government vehicles and clashed with security forces when they were stopped from moving towards a Christian church, police said.
Police dispersed the protesters using teargas and more than a dozen people were reported to be injured.
The Indian government said it was "deeply distressed" by the violence.
In a statement following a cabinet meeting on Monday, the government expressed its "profound grief at the loss of life and offers its sincere condolences to the bereaved families".
An indefinite curfew remains in place in Srinagar and other major towns in the region.
The measures were imposed after mass protests against Indian rule on Saturday again turned violent.
A plan by a Florida church to burn copies of the Koran during the 9/11 anniversary caused outrage across the Muslim world, but was eventually called off.
However, reports that pages had been torn from a Koran outside the White House over the weekend reignited the controversy and further heightened tensions in the Kashmir Valley.
Kashmir has been a flashpoint between India and Pakistan for more than 50 years.
In Delhi on Monday, the Indian cabinet resisted calls to partially lift a 20-year-old emergency law that shields their forces from prosecution.
Instead it called an all-party meeting for Wednesday to discuss solutions to the latest violence in Kashmir.
Human rights activists say the Armed Forces Special Powers Act, which also grants powerful search and seizure powers, is often misused by Indian police and paramilitaries.
Kashmir chief minister Omar Abdullah had urged the government to withdraw the act, but has met with strong resistance from the Indian military.