"It appears to me that the civilian authority is being undermined and we are fast deteriorating into a police state," Tsvangirai said.
Tensions are rising in the southern African country over policy differences between Tsvangirai’s Movement for Democratic Change (MDC) and President Robert Mugabe’s ZANU-PF.
These include ZANU-PF’s drive to nationalise the country’s mines, a move the MDC has urged caution on as the long-battered economy shows some signs of recovery.
Tsvangirai and Mugabe were forced into a coalition two years ago after a disputed poll in 2008, which led to mass violence, a flood of refugees into South Africa and a deeper economic crisis in the resource-rich and fertile country.
The slow pace of pledged democratic reforms is another a source of friction as the security forces remain staunch Mugabe loyalists, and this has led to violent clashes between supporters of the two parties.
Tsvangirai told journalists on arrival from a regional trip that he had warned leaders of the Southern African Development Community (SADC) bloc that the political violence that gripped the country in the past could flare again.
He accused security authorities of arbitrary arrests and said Zimbabwe was again in a "siege mood".
"I impressed on the SADC leaders that it is their responsibility to intervene to rescue this project (unity government) as they are the guarantors," he said.
POLICE BAN RALLY
Tsvangirai said only fresh elections would resolve the impasse. But his party would only take part if there was a SADC-backed "road map" guaranteeing free and fair elections.
Relations between the coalition rivals worsened last week when police arrested a cabinet minister from Tsvangirai’s party on suspected fraud charges and the Supreme Court nullified the election of another Tsvangirai ally as Speaker of Parliament.
Police have also arrested dozens of activists accused of plotting protests against Mugabe similar to those that toppled long-serving leaders in Egypt and Tunisia.
Tsvangirai said the police had banned a rally he planned to address on Saturday, saying they feared clashes with ZANU-PF supporters planning to hold a meeting nearby on the same day.
Police have in the last month stopped dozens of meetings organised by MDC supporters as the party restructures ahead of a five-year congress in May, Tsvangirai said.
"So many excuses have been given to try to prevent this meeting from taking place. We will do everything to make sure that this meeting continues," Tsvangirai said.
Police have previously deployed in large numbers and set up road blocks to stop MDC rallies it would have banned.