Demonstrations have been held at Burmese embassies abroad
The military authorities in Burma have ordered the indefinite closure of universities and colleges, and shut down offices of the opposition National League for Democracy.
A number of the party's key members have been arrested since the NLD's leader, Aung San Suu Kyi, was detained on Friday.
Britain, France and Sweden have demanded the immediate release of Aung San Suu Kyi, who is being held at a military guesthouse in the capital, Rangoon.
She was detained following clashes around her motorcade in the north of Burma, involving supporters of the government.
Reports say the number of people killed may be much higher than the four dead announced by the government.
The BBC's Larry Jagan says university campuses have been at the centre of political activity, and the authorities are worried that students may organise protests against Aung San Suu Kyi's renewed detention.
Most of the NLD's offices around the country have been closed by the authorities, while opposition sources say the military authorities have also begun to take into custody many of the party's key provincial leaders.
The clampdown follows clashes between NLD supporters and pro-government protesters in the north of the country on Saturday.
Aung San Suu Kyi was visiting the region at the time and was detained there.
There are fears that the extended crackdown on the opposition may mean that the generals who run Burma are planning to put Aung San Suu Kyi back under house arrest.
Aung San Suu Kyi has spent much of the last decade under house arrest
She was released from a long spell under house arrest last year - a move welcomed at the time as a sign that the junta was ready for political reform.
But she recently attacked the military for the slow pace of change and their apparent reluctance to start political talks, and tensions have been rising between the opposition and the government.
Last week, 10 members of the pro-democracy movement were sentenced to stiff jail terms for organising public protests and being involved in clandestine activities.
Our correspondent says Burmese officials have been warning Aung San Suu Kyi's supporters not to incite unrest during their travels outside Rangoon.
He says she is being blamed for clashes which occurred while she was on a political tour of the north of the country.
The military said four people were killed and 50 injured in the town of Yaway Oo, about 560 kilometres (400 miles) from Rangoon.
The NLD won elections in 1990 by a landslide, but the military regime which has run the country since 1962 refused to hand over power.
Aung San Suu Kyi has spent much of the last decade under house arrest, but her popularity among the Burmese has not waned.
After a seven-month interlude, the military junta hinted last month that it wanted to meet her again, raising expectations that the dialogue process between the two sides could be resumed.
But her detention, recent violence and last week's convictions of pro-democracy campaigners are thought to have seriously undermined those hopes.