Zimbabwean police sealed the offices of Prime Minister Morgan Tsvangirai's party on Tuesday, firing tear gas into the building and at bystanders in central Harare and raising tensions ahead of elections that could come next year.
A Reuters witness saw a truck carrying dozens of anti-riot police parked at the entrance of Tsvangirai's Movement for Democratic Change (MDC) offices while police re-routed traffic away from the area.
An unidentified small group of men was seen throwing stones at the police before the officers gave chase and fired tear gas at the group.
The police also fired tear gas to disperse hundreds of bystanders, forcing several shops to close early.
An MDC official said police were looking for an MDC member but would not give details.
The police just fired teargas into the building without warning and we were forced to escape using a back exit, the official said.
Police would not immediately comment.
President Robert Mugabe's supporters and police have barred and disrupted Tsvangirai's rallies in the western Matabeleland region in the past few weeks.
The MDC won the majority of seats in Matabeleland in elections in 2008.
Last Saturday, another MDC rally organised by a minister jointly responsible for police affairs was disrupted by ZANU-PF militants after a clash with MDC youths reminiscent of flare ups witnessed early this year.
ZANU-PF has previously denied engaging in violence and instead accuses MDC supporters of provoking its supporters.