Tanzania's main opposition presidential candidate, Tundu Lissu, on Thursday said he would not recognise the outcome of this week's election, declaring the vote "marred by irregularities at all stages".
"Whatever happened yesterday was not an election, and thus we do not recognise it. We do not accept the result," Lissu told reporters in Dar es Salaam.
"This is not an election at all. We do not accept, and do not agree, with any results coming from this process."
Lissu, from the Chadema party, is the main challenger to President John Magufuli, who is running for a second term in an election overshadowed by opposition complaints of fraud including ballot-box stuffing.
The country's polls commission has begun releasing parliamentary election results, with the opposition losing key seats, including one long-held by Chadema chairman and MP Freeman Mbowe.
Lissu, who only returned to Tanzania in July after three years abroad recovering from 16 bullet wounds sustained in what he says was an assassination attempt, said opposition election monitors were barred from observing the poll and faced other interference.
"We didn't have an election last night based on international best practice and by Tanzanian laws. It's a gang that has decided to stay in power by hook or by crook," he said.
"Democratic change is not possible in Tanzania."
Tanzania's election, for which around 29 million people were registered to vote on the mainland and 560,000 in semi-autonomous Zanzibar, which also elects its own president and lawmakers, took place largely without external monitors.