Police in Zimbabwe have raided a Harare office of the opposition Movement for Democratic Change (MDC), sources inside the country have told the BBC.
The MDC said rooms were searched but none of its officials were there.
In a separate incident police arrested two foreign journalists - one of them a reporter for the New York Times - in a raid on a hotel in the capital city.
Tensions are high with election results delayed and the MDC claiming it has defeated President Robert Mugabe.
Reports said police searched a number of hotel rooms used as offices by the MDC.
Brian Hungwe, a Zimbabwean freelance journalist in the capital, Harare, says the raid is believed to be linked to the MDC's announcement of presidential poll results ahead of the Zimbabwe Election Commission, which is illegal.
Late on Thursday, the Commission releases the first results in the election for the senate, the country's upper house of parliament.
The ruling Zanu-PF party and the MDC have five seats each. The senate has 60 seats in all.
Riot police raid
MDC secretary-general Tendai Biti denied reports that senior members of the group had gone into hiding.
He said leader Morgan Tsvangirai was "safe" but had cancelled plans for a news conference.
"You can't hide away from fascism," he said. "Zimbabwe is a small country. So we are not going into hiding. We are just going to have to be extra cautious."
The two journalists were arrested after riot police surrounded a lodge at another hotel.
The New York Times said its Pulitzer prize winning correspondent, Barry Bearak, had been arrested.
The newspaper said it did not know where he was being held, or if there were any charges against him.
The reporters were accused of practising without accreditation, police said.
A journalist who fled described a heavy police presence and said documents had been seized.
'Fight to the last'
Earlier on Thursday, President Mugabe's Zanu-PF party said he was ready to contest a second round of the presidential election.
Official results from the presidential poll have yet to be issued.
Under election rules, a candidate needs to win at least 50% of the vote to avoid a second round.
Deputy Information Minister Bright Matonga said if results from Saturday's election showed a second round was necessary, Mr Mugabe would stand.
"President Mugabe is going to fight to the last, and he's not giving up, he's not going anywhere, he hasn't lost the election," said Mr Matonga.
Zanu-PF was leading in the popular vote and if translated into a presidential vote, that would mean victory for Mr Mugabe, he said.
Zanu-PF's leadership is holding a meeting on Friday to discuss strategy. Local reporters say the party is split on whether Mr Mugabe should fight on.
Party sources are reported as saying the meeting will be chaired by Mr Mugabe and the election will be at the centre of discussions.
A BBC correspondent in Zimbabwe said there was a general sense of the nation being in limbo.
Fewer riot police are on the streets than on Wednesday, our correspondent says, but tensions remain high.
A leading Zanu-PF figure told our correspondent that members of the party's top decision-making body, the politburo, were getting distinctly nervous: "If you were them, you wouldn't want to put all your eggs in one basket," our correspondent was told.
A mid-ranking soldier told the BBC's Newsnight programme that high-level "marathon" political meetings were going on in response to the emerging results.
"I call to all juniors within the security forces to be sure they exercise maximum vigilance, because there are certain people who want to cause havoc in order to promote their own interests," the soldier said, speaking anonymously.
The opposition received a boost from the publication of official results from the parliamentary election on Wednesday.
Figures from the Zimbabwe Election Commission gave the MDC a majority of seats, displacing Zanu-PF.
Morgan Tsvangirai's MDC took 99 seats, while Mr Mugabe's party polled 97. A smaller MDC faction which backed former Zanu-PF minister Simba Makoni in the presidential polls won 10 seats leaving them with a potentially influential future role.
But in terms of the popular vote, the figures gave Mr Mugabe's party the lead - 45.9% to 42.8% for the MDC.
With presidential election results still awaited, the MDC said on Wednesday that its leader had won, though Zanu-PF denied this.
The MDC released its own results to back up its claim, saying Mr Tsvangirai had won 50.3% of the vote to Mr Mugabe's 43.8%, so avoiding a second round of voting.
But those figures differ slightly with the Zimbabwe Election Support Network, a coalition of civil society organisations, who said Mr Tsvangirai had won 49% to Mr Mugabe's 42% - which if true would necessitate a run-off.
Mr Mugabe, 84, came to power 28 years ago at independence on a wave of optimism.
But in recent years Zimbabwe has been plagued by the world's highest inflation, as well as acute food and fuel shortages.