Agathon Rwasa, Burundi's opposition leader and deputy speaker of Parliament has filed a petition at the country's constitutional court disputing the win of the ruling CNDD-FDD party's Evariste Ndayishimiye.
Mr Ndayishimiye won the May 20 presidential election with 68 per cent of the vote against Mr Rwasa's 24 per cent. The leader of CNL party filed his petition on May 28.
"We found a lot of irregularities such as votes by dead people and we have evidence," said Mr Rwasa after handing over the evidence to the constitutional court in Bujumbura.
"If the constitutional court rules in their favour I will move to the African Court because all the results that were announced by the electoral commission were wrong," said Mr Rwasa.
There were seven candidates running in the presidential race after President Pierre Nkurunziza's 15-year rule came to an end.
The ruling party was declared winner in both the presidential and legislative elections followed by the CNL and UPRONA coming third.
Despite the lack of international observer missions in the country, the government said that nine countries applied to observe the elections through their embassies in Burundi. Tanzania, Kenya, China and Democratic Republic of Congo were among them.
The country's Catholic Church deployed 2,716 observers countrywide, and has also expressed misgivings on the election process and its outcome.
"There were many irregularities in terms of liberty and transparency as some voted on behalf of refugees, while others voted more than once," said Monseigneur Joachim Ntahondereye, the chairman of the Catholic Church Conference Burundi.
Monseigneur Ntahondereye said there was intimidation of voters by some local administrators and some electoral observers had their phones confiscated.
However the chairman of the Independent Electoral Commission, Pierre Claver Kazihise, said that members of the Catholic church observer mission weren't well educated and informed about the electoral process.
Bujumbura was among the provinces where the ruling party CNDD-FDD was defeated and is known to have a large opposition support.