Bangladesh's military-backed government has imposed an indefinite curfew in Dhaka and other cities after worsening violence between students and police.
The army has retreated from the Dhaka University campus
At least one person died and 100 others were hurt as riots spread across the country, police said. The students want an end to emergency rule.
The mobile phone network has been shut down and the government has urged calm.
The violence poses the most serious challenge to the emergency government since it took power six months ago.
Clashes broke out in the capital, Dhaka, on Monday and have since spread to other parts of the country, including Sylhet in the north and the port of Chittagong in the south.
In all, five main cities plus the capital have been placed under overnight curfew, starting from 2000 (1400 GMT).
Late on Wednesday, the head of the caretaker government, Fakhruddin Ahmed, addressed the nation.
He said "evil forces" and opportunists were trying to foment anarchy, but he promised the curfews were only a temporary measure.
Universities and colleges in the six cities have been ordered to close and all students told to leave. Television stations have been ordered to stop broadcasting footage of the violence.
Correspondents say the protests appear to be growing more violent, and are turning into a popular movement demanding the immediate restoration of democracy.
Slum dwellers, shop keepers, rickshaw pullers and businessmen joined stone-throwing students in violent and bloody demonstrations in Dhaka. They confronted police who baton charged them and fired tear gas.
The BBC's John Sudworth at Dhaka University said armoured personnel carriers could be seen announcing the curfew to hundreds of protesters manning barricades, lighting fires, throwing stones or just trying to get back home.
The student protests appear to be growing
The first casualty of the unrest was a rickshaw puller killed during rioting in the north-western town of Rajshahi, officials said.
Police are accused of using tear gas indiscriminately, at one point on Tuesday even firing it into a female students' dormitory at Dhaka University to prevent them from joining the protests.
Monday's clashes began when students in Dhaka said they had been "manhandled" by soldiers during a football match.
Violence continued even though a student demand that troops be withdrawn from the Dhaka campus was met early on Wednesday.
The police rather than the army are confronting the protesters
The students have also demanded a public apology from the army chief, Gen Moeen U Ahmed, and want soldiers who have beaten up students to be punished.
The violence is in contrast to widespread popular support for the interim government when it came to power in January promising to stamp out corruption and to hold elections by the end of 2008.
But discontent has been rising in recent months, most notably over the rapidly rising cost of living.
Two other military governments in the past - that of Ziaur Rahman and Muhammad Ershad - were both brought down after protests that were started by students.