Lilongwe, Malawi - Opposition parties in Malawi have hailed a unanimous ruling by the country's constitutional court which on Monday ordered a new presidential election after annulling the disputed results of last year's presidential vote.

The long-awaited decision sparked scenes of celebration among opposition supporters in the capital, Lilongwe, and other parts of the country.

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The Malawi Electoral Commission (MEC) had declared President Peter Mutharika the narrow winner of an election in May of last year, with 38 percent of votes, followed by Lazarus Chakwera, with 35 percent, and former Vice President Saulos Chilima in third place, with 20 percent. The top two contenders then petitioned the court to have the results nullified, alleging several irregularities.

In their 500-page ruling, the panel of judges cited the widespread use of unauthorised correction fluid to alter figures, the use of duplicate result sheets and unsigned results forms as cases that compromised the outcome of the vote.

They said the results announced by MEC "cannot be trusted as a true reflection of the will of the voters" and told Parliament to evaluate whether the commission can conduct the new election, which should be held within 150 days.

"We would hope that everyone will play a part in order to make this a reality," said Chakwera, who had also complained of being robbed of victory in the 2014 election.

"This is a landmark decision for Africa, for Malawi, for democracy and we would want to see that everyone respects that," he added.

In an interview immediately after the ruling, Attorney General Kalekeni Kaphale, who represented MEC, said they will examine the judgment and decide on the next course of action.

The ruling can be appealed at the Supreme Court.

There was no immediate comment by Mutharika.

'No iota of credibility'

The court further ordered that the new elections be held under a majoritarian system, a legal provision which was rejected by Parliament - which instead opted for simple majority system - more than two years ago.

Since the announcement of the election results over eight months ago, Malawi has experienced a series of protests across the country demanding the resignation of senior MEC members for allegedly mismanaging the vote.

Following Monday's ruling, the Human Rights Defenders Coalition, which helped organise the protests, threatened to resume their demonstrations unless there were changes at the top of the commission.

"We think when we leave it in the hands of politicians, nothing works. We will use our own means to make sure these people are taken to account," Timothy Mtambo, the coalition's chairman, told Al Jazeera.

Blessings Chinsinga, professor of political science at the University of Malawi, said MEC's senior officials should step aside following the ruling.

"I am surprised that they have not voluntarily resigned," he said.

"They do not have any iota of credibility and they should never be anywhere near Malawi's electoral processes. They have let the nation down and ... must face the law so that we do not have such incident in the future," Chinsinga said.

Echoing the calls for accountability, Garton Kamchedzera, professor of law at the University of Malawi, stressed that a president should not be involved in the appointment of the electoral body's commissioners.

"One would not expect the current president, who saw nothing wrong in how MEC conducted those elections, to be the same person to be making appointments or to proceed with the same modalities," he said.

"One would expect that the reforms would start from there."