A new vote will be held within 150 days, the court said in Monday's unanimous ruling, adding that it hoped the ruling would not "destroy the nation".

The two leading opposition candidates had challenged the narrow election win of President Peter Mutharika, alleging that irregularities affected over 1.4 million of the total 5.1 million votes cast.

Months of sometimes deadly unrest followed the announcement of the election results.

The president and electoral commission acknowledged some irregularities but argued they were insufficient to affect the election's outcome.

Monday's ruling can be appealed to the Supreme Court.

The president's legal team refused to answer questions and hurriedly left the court premises.

The attorney general, representing the electoral commission, said they would have to consult on next steps.

Security was tight and people across the country followed the day-long court session, read out in English and Chichewa, live on radio broadcasts.

Many listened spellbound as the court listed multiple irregularities, from the liberal use of the correction fluid Tipp-Ex that "greatly undermined" the vote's integrity to the lack of signatures on some results forms.

The court told the country's parliament to evaluate whether the electoral commission can conduct the new election.

The judges even challenged the electoral system, saying the results suggested no one was elected by a majority in accord with the Constitution.

It ordered parliament to meet within 21 days to come up with a new law to guide the fresh elections.

Australian Associated Press