The East African Community observation mission in Burundi has said the Tuesday presidential election was neither free nor fair.
This comes hours before the country's elections commission CENI is expected to release the results which President Pierre Nkurunziza is widely expected to win following an opposition boycott.
A preliminary statement from the observer team, which cancelled a press briefing Thursday, says the election fell short of "the principles and standards for holding free, fair, peaceful, transparent and credible elections".
Western countries including, the African Union and the European Union, withdrew their observers last month following increased political violence leading to over 150,000 Burundians fleeing to neighbouring countries in the Democratic Republic of Congo, Tanzania and Rwanda.
But the UN, the International Conference on the Great Lakes region (ICGLR) and neighbouring countries such as Kenya, Tanzania and DRC sent in elections observers.
On the eve of the poll, violence broke out with incidents of shootings and grenade attacks reported in the capital Bujumbura "which heightened fear among the population".
Violence forced the mission not deploy observers in neighbourhoods like Cibitoke which have been the scene of fierce fighting between the security forces and the opposition, the observer team says.
Even though President Nkurunziza is widely expected to win, his legitimacy is likely not just to be contested by the opposition, but by Western countries as well who have made it clear his new mandate is illegal.
Last week, the opposition announced the formation of a transitional council that will bring together all anti-Nkurunziza voices in a new bid to unseat the president.
A summit is expected next week in Addis Ababa, Ethiopia where the wider opposition coalition will discuss to elect its top leadership and start initiating contact with Western capitals for diplomatic recognition.
"We are going to put in place political and diplomatic committees to drive our agenda domestically and abroad," Alexis Sinduhije, the leader of the opposition party Movement for Solidarity and Development (MSD), told The EastAfrican.
The EU and the US have also announced plans to impose sanctions on high ranking government and party figures seen to be frustrating efforts at finding a peaceful resolution of the conflict.
"The European Union is preparing ... to adopt, if necessary, targeted restrictive measures against those whose actions led to acts of violence, repression and serious human rights abuses or hinder the search for a political solution," said Federica Mogherini, the EU's foreign policy chief.
At least six government officials are likely to be targeted with asset freezes and travel bans, according to EU officials.
US has also threatened to review Burundi's eligibility to Agoa which could see the country lose up to $4 million worth of foreign exchange annually.
The EAC observer team further called for continued engagement with all parties "to find a sustainable solution to the prevailing political impasse in the country".