Police raided a hotel used by the opposition Movement for Democratic Change and ransacked some of the rooms. Riot police also surrounded another hotel housing foreign journalists and took away several of them, according to a man who answered the phone there.
"Mugabe has started a crackdown," Movement for Democratic Change secretary-general Tendai Biti told The Associated Press. "It is quite clear he has unleashed a war."
The New York Times said that its correspondent Barry Bearak was taken into custody by police.
"We do not know where he is being held, or what, if any, charges have been made against him," said Bill Keller, executive editor of the Times. "We are making every effort to ascertain his status, to assure that he is safe and being well treated, and to secure his prompt release."
Foreign journalists have been in Zimbabwe to cover elections in which President Robert Mugabe's party lost control of parliament. He is apparently facing a runoff for the presidency.
Biti said the raid at the Meikles Hotel targeted "certain people ... including myself." Opposition leader Morgan Tsvangirai was "safe" but had canceled plans for a news conference, Biti said.
He said Thursday's clampdown was a sign of worse to follow but that the opposition would not go into hiding.
Mugabe is ready for a runoff, Deputy Information Minister Bright Matonga said, as the opposition claimed it won the presidential race outright and official results showed it won the majority of seats in the 110-member Parliament.
Results for the 60-member Senate, however, have been delayed, the Zimbabwe Electoral Commission said, citing "logistical problems." The commission said the results would be announced as soon as they are verified.
Mugabe was said to be pondering conflicting advice on whether to cede power or face a runoff - a humiliating dilemma for a man who has ruled for 28 years. On Thursday, Mugabe, making his first public appearance since the elections, was shown on state television meeting African Union election observers.
"President Mugabe is going to fight. He is not going anywhere. He has not lost," Matonga told the British Broadcasting Corp. "We are going to go hard and fight and get the majority required."
Independent observers say their own projection - based on results posted at a representative sample of polling stations - showed opposition leader Morgan Tsvangirai winning the most votes but not enough to avoid a runoff, which must be held within 21 days of the first round.
International concern about the continuing delays mounted Thursday.
"We still have not seen the important thing, which is real live election results," said U.S. State Department spokesman Tom Casey. "We need to see an official tally, see it soon and have assurances made that this is actually a correct counting of the votes."
"Delays raise serious questions in our minds about what is going on in the vote counting," he said.
An electoral commission member indicated that the presidential results would be announced Friday. He spoke on condition of anonymity because he was not authorized to speak to the media.
The commission said it still was receiving ballot boxes from the provinces, raising questions about where those votes have been since Saturday's elections amid charges of a plot to rig the results. Western election observers have accused Mugabe of stealing previous elections.
Official results for parliament gave the ruling ZANU-PF 1,112,773 of votes to 1,038,512 for Tsvangirai's party and 203,146 for Mutambara's faction. An independent won 54,259 votes and smaller numbers went to parties that won no seats.
A total of 2.4 million valid votes were cast, according to the figures, supporting opposition charges that the voters' roll of 5.9 million was hugely inflated with names of dead and fictitious people and some of the estimated 5 million Zimbabweans who have become political and economic refugees abroad.
The state-run Herald warned that Tsvangirai would hand back farmland to the whites if he becomes president. Tsvangirai has not said that, instead promising an equitable distribution of land to people who know how to farm.
Mugabe claimed his land reforms were to benefit poor blacks, but gave most seized farms to relatives, friends and cronies, with some senior officials and military commanders receiving several fertile farms that have been overtaken by weeds.
The Herald said white farmers had returned from Zambia and Mozambique and were threatening to evict blacks. It quoted the war veterans association that spearheaded violent land grabs as saying "We will be left with no option except to take up arms and defend our pieces of land."
War veteran Garikai Sithole urged Zimbabweans to "avoid aborting the revolution at this critical stage," according to the newspaper.
Sithole said people were blaming Mugabe for their woes, but: "When you cannot maintain your family and you turn around and say, 'It's Mugabe's fault,' this is just hiding from the truth."
Mugabe blames former colonizer Britain and other Western nations for the collapse of Zimbabwe's economy. Targeted Western sanctions, though, only involve visa bans and frozen bank accounts for Mugabe and about 100 of his allies.
Mugabe calls opposition leaders stooges and puppets of Britain. The Herald said "the British government and Prime Minister Gordon Brown have now come out in the open as the real power" behind Tsvangirai and his Movement for Democratic Change.
Religious leaders and diplomats were involved in a flurry of initiatives Thursday to try to persuade Mugabe to step down. Diplomats who spoke on condition of anonymity because of the sensitivity of the issue said Western leaders were contacting southern African leaders. Amani Countess of the Washington-based TransAfrica Forum said religious leaders were asking counterparts in the region to pressure presidents to approach Mugabe.
Mugabe has ruled since his guerrilla army helped force an end to white minority rule in then-Rhodesia and bring about an independent Zimbabwe in 1980. But the man hailed as a liberator has become increasingly despotic and overseen the destruction of a thriving economy in the eight years since he ordered the often-violent seizures of vast tracks of commercial farmland owned by a few thousand whites.
Still, about half of Zimbabweans who voted in weekend elections marred by some electoral irregularities and intimidation chose the ruling ZANU-PF party, though a third of the population in this former food-exporting nation now depends on international handouts and 80 percent is jobless.