In response, Zimbabwe's opposition party is asking the courts to force the release of presidential election results.
Party secretary and Minister of State Didymus Mutasa charged Friday that the opposition bribed electoral officials and said his party would contest results of 16 parliamentary seats - the party had lost its parliamentary majority, according to official results from Saturday local council, legislative and presidential voting.
"We agreed to have a rerun at a date to be set by" the electoral commission, Mutasa said at a news conference after a five-hour party politburo meeting, the first since the elections. Diplomats have said Mugabe may try to put the runoff off for three months.
While official presidential election results have not yet been released, independent observers had projected a runoff, saying Tsvangirai won the most votes, but not the 50 percent plus one vote necessary for an outright victory. The opposition had claimed to have won the presidency outright, but also said it would contest a runoff if one were ordered.
Earlier Friday, police escorted about 400 war veterans as they paraded silently through downtown Harare. The feared veterans in the bush war that helped end white minority rule, who often are used to intimidate opposition supporters and spearheaded the often violent takeover of white farms in recent years, appeared to have been transported to town.
At a news conference, Jabulani Sibanda, head of the Zimbabwe War Veterans' Association, said ZANU-PF lost the elections because "people were pushed by hunger and illegal sanctions. Under current circumstances the sprit of our people is being provoked. We will be forced to defend our sovereignty."
At another news conference Friday, outspoken Mugabe critic Lovemore Madhuku, who in the past has suffered police beatings and an arson attack on his home, urged Zimbabweans to "defend their vote."
Madhuku called on Zimbabweans "not to give in to any form of intimidation, any form of violence ... that would be used to stop change or to steal the vote."
The opposition said it was ready to go to court to force the Zimbabwe Electoral Commission to release results, pointing out that the law gives the election commission one week to release all results.
The opposition has been weakened by internal divisions. But Friday, a splinter faction said divisions weren't an issue should there be a runoff.
"Whatever formation is there to remove Mugabe, we are there to support it," said Abednico Bhebhe, spokesman for the Arthur Mutambara faction of the MDC.
The law requires a runoff within 21 days of the first round. But Diplomats in Harare and at the United Nations said Mugabe was planning to declare a 90-day delay to give security forces time to clamp down.
In ominous signs overnight, intruders ransacked offices of the main opposition party and police detained foreign journalists.
"Mugabe has started a crackdown," MDC secretary-general Tendai Biti told The Associated Press, saying rooms used as offices by the opposition at a Harare hotel were ransacked Thursday by intruders he believed were either police or agents of the feared Central Intelligence Organization.
Biti said Tsvangirai was "safe" but had canceled plans for a news conference.
The journalists, meanwhile, were detained by heavily armed riot police who surrounded and entered a Harare hotel frequented by foreign reporters, lawyers said. The U.S.-based National Democratic Institute added one of its staff, an American, was detained by authorities at Harare's airport as he tried to leave the country Thursday.
The government had rejected most foreign journalists' applications to cover the elections, and had barred Western election observers.
Sky News reported Friday that two of the reporters had been charged with practicing journalism without licenses. The other three were released.
Bill Keller, executive editor of The New York Times, said Pulitzer Prize-winning correspondent Barry Bearak was among the reporters initially detained, but it was unclear whether he was one of the two reporters charged Friday.
Speaking to Sky News, Mugabe's Deputy Information Minister Bright Matonga said Friday, "these journalists need to prove that they have been accepted (into the country)" and he accused Western reporters of trying to "inflame a situation that doesn't need fire."
"We are very keen on foreign journalists operating in Zimbabwe," Matonga said, "but you wanted to barge in, you wanted to push yourselves in... So we had to take a stand".
Sky's Emma Hurd, one of the Western journalists who was still broadcasting from inside Zimbabwe on Friday without a permit to be there, said her organization had applied for years to work inside the country but was consistently denied entry.
The New York-based Committee to Protect Journalists expressed alarm and called for the detained reporters' immediate release.
On Friday, lawyer Harrison Nkomo said the attorney general had said there was no case against the two foreign journalists and that is was up to police to release them or put new charges to them.
He said two of his colleagues were at the Harare central police station "to facilitate their release."
Mugabe has ruled since his guerrilla army helped force an end to white minority rule and bring about an independent Zimbabwe in 1980. His popularity has been battered by an economic freefall that followed the often-violent seizures of white-owned commercial farms in 2000.
Zimbabwean authorities on Friday introduced a new 50-million bank note, state media reported. The new Zimbabwe dollar note is worth US$1 (about 60 euro cents) at the widely used black market trading and can buy just three loaves of bread.