Mr Muchadehama said the MDC would do its best to get the petition heard
Armed police in Zimbabwe have prevented lawyers from the opposition MDC from entering the high court to file a petition on the presidential elections.
Lawyer Alec Muchadehama said the police had threatened to shoot them.
The MDC wants the court to force officials to release the result of last Saturday's poll. It says its leader Morgan Tsvangirai won the ballot.
His opponent President Robert Mugabe's Zanu-PF party has said it will back him if a run-off is called.
Reuters news agency said the police apparently came from presidential offices across the street, telling the lawyers no-one could enter the court.
Another opposition lawyer, Andrew Makoni, later said the hearing - originally scheduled for 1200 local time (1000 GMT) - had been postponed until Sunday.
Zimbabwean Deputy Information Minister Bright Matonga accused the MDC (Movement for Democratic Change) of spreading false and malicious reports to get international sympathy.
He said police had probably been trying to contain the party's rowdy supporters.
The MDC has also called for international help to prevent possible violence if a second round is held.
But speaking in the UK, South African President Thabo Mbeki said that now was not the time to interfere, and the international community should wait to find out the outcome of the election.
On Friday, Zanu-PF leaders backed Mr Mugabe's participation in a possible run-off.
There had been speculation he would stand aside rather than face a second poll.
Correspondents say there are fears a second round - which would be expected to take place within three weeks - could lead to a resurgence of the violence and intimidation that has been a characteristic of past elections in Zimbabwe.
Announcing the legal challenge, another MDC spokesman, Nelson Chamisa, said the delay in reporting the result was unjustified.
"We're fighting the anxiety, disappointment, speculation and rumours as a result of this delay," he said.
Mr Chamisa said the ruling party would attempt to use a run-off as a way to exact revenge, and called for immediate international help.
"For Mugabe, a run-off is a strategy for retribution," he said.
"He was beaten at the referendum in 2000 and we all know the violence which followed.
"This is what we want to avoid, rather than have the international community intervene after there has been bloodshed."
Harare is still covered in election posters
Zanu-PF said it would call for recounts for 16 seats in the parliamentary elections.
If successfully contested, these would be enough for the party to regain the majority it lost for the first time since 1980.
The MDC says its candidate, Morgan Tsvangirai, took 50.3% of the vote, just over the 50% needed to avoid a run-off.
An independent projection says Mr Tsvangirai gained 49%, just below the threshold, with Mr Mugabe on 42%.
On Friday, hundreds of Zanu-PF supporters - some of them veterans from the war against white rule that led to independence - marched through the capital, Harare, the Associated Press news agency reported.
Jabulani Sibanda, head of the Zimbabwe War Veterans' Association, which has been associated with past election violence, said Zanu-PF had lost the elections because "people were pushed by hunger and illegal sanctions".
"Under current circumstances the spirit of our people is being provoked," he said.
"We will be forced to defend our sovereignty."
Western countries imposed sanctions following allegations that Mr Mugabe rigged the polls in 2002.
The sanctions are targeted at Mr Mugabe and his close associates - they are subject to a travel ban and an assets freeze in the European Union and the US.
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, which correspondents say have driven many voters to back the opposition.
The ruling party remains divided, the BBC's Peter Biles says, with many who would still like to see a change of leadership, believing that under Mr Mugabe, Zimbabwe has no future.
So far, 10 of the 60 Senate results have been officially announced, with each party taking five seats.
In the lower house of parliament, the MDC took 99 seats, while Zanu-PF won 97.
A smaller MDC faction, which backed former Mugabe loyalist Simba Makoni in the presidential election, won 10 seats, leaving them with a potentially influential role.
However, Zanu-PF gained 46% of the vote in the parliamentary race, against 43% for the MDC, which supporters of Mr Mugabe say gives him hope of victory in a run-off.