Wayne Bvudzijena, a national police spokesman, told AFP that he could confirm the arrests of only two, although lawyers said five journalists were taken away from the hotel.
The identities of the other reporters have not been determined.
Beatrice Mtetwa, a Zimbabwe lawyer, said the two journalists were told they would be charged on Friday while the other three were released.
Tendai Biti, the MDC secretary-general, said: "Mugabe has started a crackdown.
"It is quite clear he has unleashed a war."
He said government agents had raided hotel rooms the MDC had been using as offices, but Al Jazeera could not verify the complaint.
Police were manning a number of roadblocks in the capital on Thursday, but there was no other sign of an overt security presence.
Delay and crackdown
Mugabe appeared on state television on Thursday for the first time since the elections, meeting Ahmad Tejan Kabbah, the former president of Sierra Leone, who is heading an African Union election observer team.
The Zimbabwean president is said to be pondering conflicting advice from his advisers on whether to quietly cede power or face a run-off.
Both are humiliating prospects for the 84-year-old.
Independent election observers say their projections, based on election results posted at a representative sample of local polling stations, indicate Morgan Tsvangirai, leader of the MDC, had won the most votes in the presidential contest, but not enough to avoid a run-off.
|Tsvangirai has refrained from declaring himself |
president so far [GALLO/GETTY]
The law requires a run-off be held within 21 days of an election, which would be by April 19, but Mugabe could change that with a presidential decree, a Western diplomat in Harare said.
But diplomats in Harare and at the United Nations said Thursday's events indicated he might be considering a third option: declaring a state of emergency and 90-day delay to the presidential run-off to give government forces time to suppress the opposition.
Reports said leaders of the ruling party scheduled a meeting on Friday to discuss the run-off.
Didymus Mutasa, Zanu-PF secretary for administration, said: "All I can confirm is there is a politburo meeting. That's enough, that's all I can say at the moment."
Delays in the announcement of election results had prompted earlier rumours that Mugabe might step aside.
But Bright Matonga, the deputy information minister, dismissed rumours that Mugabe was considering stepping aside and indicated there would be a second round of voting.
"President Mugabe is going to fight. He is not going anywhere. He has not lost. We are going to go hard and fight and get the majority required," he said.
"We think, and it is my assumption ... there may not be a clear winner of the presidential one [vote] and it points to a re-run.
"From Zanu-PF's perspective, we are very confident that we've got the numbers, when it comes to a re-run, we're ready for that second round, and we are confident that President Robert Mugabe will win this time."
Seemingly laying the groundwork for a Mugabe run-off campaign, the state-run Herald newspaper said the ruling Zanu-PF party was running neck-and-neck with the opposition in the vote count, and it highlighted divisions among Mugabe's foes.
The newspaper also charged that Tsvangirai would give farmland back to whites.
The opposition leader has not said that, but has promised to make an equitable distribution of land to people who know how to farm.
Five days after the vote, the Zimbabwe Electoral Commission still has not released results on the presidential election despite increasing international pressure, including from Kofi Annan, the former UN secretary-general, who recently mediated an end to Kenya's post-election violence.
The opposition won the constituency-based parliamentary poll, but no results have yet been released for the presidential vote.
The MDC has already asserted that Tsvangirai won the presidency outright based on its own tallies, but said it was prepared to compete in any run-off.
Biti said Tsvangirai had won 50.3 per cent against Mugabe's 43.8 per cent and urged Mugabe to concede defeat and avoid embarrassment.
Despite his party's proclamation of victory, Tsvangirai has refrained from declaring himself the president, a move seen as having helped prevent major unrest in Zimbabwe.
Tsvangirai tried on Thursday to reassure security chiefs who vowed a week ago not to serve anyone but Mugabe, according to a source close to the opposition leader.
But an agreed meeting with seven generals was cancelled when the officers said that they had been ordered not to attend and that they would be under surveillance, the source said.