Two Zimbabwean policewomen are in hospital with serious facial injuries after their Harare base was fire-bombed, police say.
Mr Tsvangirai says his beating should be an inspiration to activists
A police spokesman blamed the attack on the "armed wing" of the opposition Movement for Democratic Change and said force would be used against them.
The reported attack comes as MDC leader Morgan Tsvangirai remains in hospital after allegedly being beaten by police.
Western nations have called for more sanctions on Zimbabwean officials.
Assistant Commissioner Wayne Bvudzijena said that a police camp in the central city of Gweru was also fire-bombed on Wednesday night.
"We are seeing militia-style attacks by an armed wing of the MDC," he told the AFP news agency.
He warned that the police would meet such violence with the "necessary force to ensure peace and stability is maintained".
Zimbabwe's information minister has said the MDC would "pay a heavy price" for inciting violence and said Mr Tsvangirai's injuries were sustained when he and others attacked police during a political meeting last Sunday.
The MDC leader denies being involved in any violence and said his beating should be an "inspiration" for the struggle against President Robert Mugabe.
Scores of other MDC officials and activists were arrested and allegedly assaulted after police broke up a rally on Sunday.
One man was shot dead.
South African pressure
Meanwhile, Tanzania's President Jakaya Kikwete has gone for an unscheduled meeting with Mr Mugabe to discuss the situation in Zimbabwe.
Tanzania has traditionally been a close ally of Zimbabwe's long-time leader.
Mr Mugabe and top government and ruling party officials are already subject to a travel ban and assets freeze by the US and the EU but these may now be extended.
UK Prime Minister Tony Blair described the situation as a tragedy for the people of Zimbabwe.
"They should be able to express their political views without harassment or intimidation or violence. And what is happening in Zimbabwe is truly tragic," he said.
Mr Blair was speaking after a meeting with Ghanaian President John Kufuor, who is currently chairman of the African Union.
Mr Kufuor said African leaders were embarrassed by the situation in Zimbabwe but rejected suggestions that they were unconcerned by the violence.
The action against Sunday's demonstration was also strongly condemned by the UN Secretary-General, Ban Ki-moon.
But the most unexpected criticism came from South Africa, which urged Mr Mugabe to respect the rights of citizens, including opposition leaders.
BBC Africa editor Martin Plaut says that after years of fending off international pressure, Pretoria has adopted an entirely new tone.
South Africa is the regional powerhouse and analysts believe it is the country best placed to put pressure on Mr Mugabe to either step down or do more to respect human rights and change his economic policies.
Mr Mugabe has said he wants to change the constitution to remain in office until 2010, even though his term of office is due to expire next year.
Civil discontent in Zimbabwe is rising over the country's economic crisis, with chronic unemployment and inflation running at more than 1,700% - the highest in the world.