Tanzania's main opposition candidate said he would not accept the general election results and alleged fraud.
Tundu Lissu alleged on Thursday there were irregularities, including ballot-box stuffing in Wednesday's vote. He has called on supporters to protest peacefully.
Lissu was one of 14 candidates challenging President John Magufuli, whose CCM party has been in power since 1961. Right's groups have decried a slide into autocracy in the last several years.
"We will not accept anything coming out of yesterday's voting because it was marred by irregularities at all stages, " Lissu said.
As vote counting continues on the mainland, Tanzania's electoral commission said Magfuli won 85 percent of the 32 constituency results. The semi-autonomous archipelago of Zanzibar, which elects its own president and lawmakers, announced the ruling Chama Cha Mapinduzi (CCM) presidential candidate Hussein Ali Mwinyi as the victor.
'Not free and fair'
Even before the vote, there were reports of violence by the opposition and concern about the fairness of the election.
"I as a bishop and all the other bishops that stand for justice, I declare openly that the elections held in Tanzania were not free and fair," said Emmaus Bandekile Mwamakula, Bishop, Moravian Revival Church.
Lissu, 52, returned to Tanzania in July after recovering from 16 bullet wounds from what he believes were from a politically-motivated assassination attempt.
His return reinvigorated an opposition demoralised by years of attacks, arrests and a ban on political rallies, with massive crowds seen throughout his campaign.
However several opposition MPs lost seats in long-held bastions, such as Chadema chairman and lawmaker Freeman Mbowe of Hai in the Kilimanjaro region and ACT-Wazalendo party leader Zitto Kabwe in Kigoma on the shores of Lake Tanganyika.
Of 264 parliamentary seats up for grabs, the CCM has so far won 128, losing only one to the small Civic United Front Party.
A statement from the US embassy said "irregularities and the overwhelming margins of victory raise serious doubts about the credibility of the results" and the government's "commitment to democratic values".