The leader of Tanzania's main opposition party Chadema has demanded a re-run of Sunday's presidential poll.
Willibrod Slaa told the BBC the results being announced by the electoral commission were different from those counted in the constituencies.
But the commission head dismissed his concerns, saying there may be minor irregularities, but not enough to change the final result.
International observers have expressed concern about the count and delays.
On Monday, opposition supporters held protests in several parts of the country at the slow pace of announcing the results.
Correspondents say incumbent President Jakaya Kikwete is expected to win the presidential poll, although Mr Slaa has mounted a stronger challenge than previous opposition leaders.
Meanwhile, on the semi-autonomous archipelago of Zanzibar, thousands of islanders turned out to witness the swearing-in of President Ali Mohamed Shein of Mr Kikwete's Chama Cha Mapinduzi (CCM) party.
He beat Seif Sharif Hamad, of the opposition Civic United Front, by fewer than 4,000 votes.
Under a previously agreed power-sharing deal, Mr Sharif will serve as one of Mr Shein's vice-presidents.
The agreement was aimed at bringing to an end the violence that erupted during Zanzibar's polls in 2000 and 2005.
Mr Slaa said the tallies at polling stations differed from those announced by the electoral commission.
He called on the commission to stop announcing the results.
"We don't have much time. According to the law in Tanzania, the moment the president is declared a winner there is no way you can challenge it in any court of law or anywhere else," he told the BBC's Focus on Africa programme.
"Suspend results of all the presidential elections and restart the whole process afresh."
But the National Electoral Commission chairman rejected the idea.
"There could be irregularities in terms of arithmetic in the vote tallying, but not enough to change the final results," Lewis Makame is quoted by the Reuters news agency as saying.
He went on to announce more results, which showed Mr Kikwete on course for a second term in office.
Mr Kikwete has been credited with boosting the nation's economy, but his opponents say he has failed to tackle widespread poverty.