In Congo: Opposition says its tally shows incumbent lost presidential vote

"It is impossible that the candidate-president could claim a victory, even taking into account only the partial results," the coalition said in a statement.

  Congo Republic President Denis Sassou Nguesso attends the closing rally of his electoral campaign ahead of Sunday's presidential election, in Brazzaville March 18, 2016. Picture taken March 18.  

Opposition candidates in elections in Congo Republic said on Wednesday that President Denis Sassou Nguesso placed no better than fourth in any major district, rejecting official partial results that gave him a commanding lead.

Results of Sunday's ballot announced by the country's electoral commission on Tuesday, and based on returns from 72 of 111 voting districts, showed Sassou Nguesso with 67 percent of the vote .

Charles Zacharie Bowao, the head of a coalition of five opposition candidates, posted its summary of preliminary results -- showing Sassou Nguesso trailing others -- on his Twitter account.

It said this was based on results collected from regions representing more than 75 percent of the electorate, though no exact vote totals were provided.

"It is impossible that the candidate-president could claim a victory, even taking into account only the partial results," the coalition said in a statement.

Bowao did not say which of the opposition candidates the summary placed in the lead, or how it was compiled. The opposition previously said it had deployed observers to all polling stations to tally votes.

Sassou Nguesso, who has ruled the oil-producing country for 32 of the last 37 years, must win an outright majority against eight opponents in all to avoid a run-off.

The government says the opposition's publication of its own results is illegal and has imposed a blackout on internet and mobile phone communications since Sunday that it says will help prevent unofficial results circulating and creating unrest.

The U.S. State Department said that it had "received numerous reports of irregularities that have raised concerns about the credibility of the process", urging authorities in a statement to restore communications.

Final results from the electoral commission are expected as early as Wednesday afternoon amid heightened tensions in southern Brazzaville, an opposition stronghold.

Police fired tear gas on Wednesday morning to disperse opposition supporters who had gathered outside one candidate's campaign headquarters to await the results, witnesses said.

There was a also a heavy police and military presence in the economic hub Pointe Noire, where the opposition is well-supported, a resident told Reuters.

At least 18 people were killed by security forces in protests ahead of a referendum in October that removed term and age limits that would have prevented Sassou Nguesso from running again.

Congo's election is being watched closely across Africa, where several long-ruling presidents are trying to remove constitutionally mandated term limits to stay in power.