Zimbabwe President Robert Mugabe's government vowed to silence dissent on Wednesday, defying international outrage over an assault which left the opposition leader with a suspected fractured skull.
Images of battered Mugabe critic Morgan Tsvangirai going to court after his arrest on Sunday have brought condemnation from several countries including the United States.
Washington said on Wednesday it was looking at what additional sanctions it might impose on Zimbabwe.
Mugabe, Zimbabwe's sole ruler since independence from Britain in 1980, showed no sign of softening his approach.
Political tensions have increased in recent weeks because of plans by Mugabe, now 83, to further extend his rule.
His government accused Tsvangirai, the head of the Movement for Democratic Change (MDC), and his supporters of inciting violence to overthrow the administration and warned their campaign would end in "grief".
"Those who incite violence, or actually cause and participate in unleashing it, are set to pay a very heavy price, regardless of who they are," Information Minister Sikhanyiso Ndlovu said in the statement.
Police surrounded the headquarters of Tsvangirai's party in Harare late on Wednesday evening when it had no staff in it, an opposition official said.
"They have barricaded the building and have four trucks in front of the building but we are not sure what they want to do," said Luke Tamborinyoka an information officer with Tsvangirai's Movement for Democratic Change (MDC).
The MDC said police also arrested two executives from the party's offices in the southern town of Bulawayo.
There was no immediate comment from the police.
The United States has condemned police action against Tsvangirai and his followers as "ruthless and repressive" and regional powerhouse South Africa, which normally avoids direct comment on Zimbabwe's woes, called on Mugabe's government to respect the rule of law.
Tsvangirai, speaking to a radio reporter from his hospital bed, said he was attacked after arriving at a police station to check on supporters who had been detained with him on Sunday when the government broke up a planned prayer vigil.
"It was almost as if they were waiting for me," he said in remarks broadcast on South Africa's national radio.
"Before I could even settle down I was subjected to a lot of beatings, in fact it was random beatings, but I think the intention was to inflict as much harm as they could."
Tsvangirai's spokesman said he was in intensive care.
"He has just had a brain scan because his skull is cracked," spokesman William Bango said from Tsvangirai's Harare hospital, adding the opposition leader had also needed blood transfusions.
African Union Chairman John Kufuor said African leaders were embarrassed by the situation in Zimbabwe and perhaps could do more to help, but have met stiff resistance from Harare.
"The African Union is very uncomfortable. The situation in your country is very embarrassing," Ghana's president Kufuor said in response to a question from a Zimbabwean at the Chatham House think-tank during a state visit in London.
British Prime Minister Tony Blair said after meeting Kufuor the situation in Zimbabwe was "truly tragic".
The latest crackdown comes as Zimbabwe faces a deepening economic crisis with inflation at more than 1,700 per cent, unemployment of 80 percent and shortages of food, fuel and foreign exchange.