HARARE, Zimbabwe (AP) — Zimbabwe’s electoral panel on Saturday declared that longtime President Robert Mugabe had won re-election by a landslide, a result that could exacerbate tensions in the country, where the 89-year-old’s chief rival and former coalition partner has accused him of poll-rigging.

Mugabe seemed set to strengthen his hold over Zimbabwe after the state Election Commission said his party won 158 of the 210 parliament seats. That gives it a two-thirds majority in the legislature — enabling it to amend a recently approved constitution that provides for democratic reforms.

Challenger Morgan Tsvangirai’s party, which had gambled that a high turnout in its favor would overcome any alleged fraud in the vote, captured 50 seats and two went to independent candidates.

According to the results, Mugabe won 61 percent of the vote, compared to 33 percent for Tsvangirai, who had been prime minister in a tense power-sharing deal with the president. Officially, Mugabe, who has been in power for 33 years, gets another five-year term in office.

Tsvangirai rejected the results as fraudulent and called for fresh elections. He urged a peaceful response to the alleged massive rigging by Mugabe’s ZANU-PF party, which has the muscle of the security forces to deter any groundswell of street protests.

In contrast to an election marked by deadly attacks in 2008, the vote on Wednesday was mostly peaceful and African poll monitors, while expressing some concern about reported irregularities, seemed mostly relieved that it was not violent.

Britain and the United States were more forceful in their criticism of the voting process, though Mugabe has based his career in part on sparring with Western powers and there is little chance their disapproval will sway him.

That leaves the Zimbabwean opposition with few options for countering Mugabe, who presides over a country that still has economic problems but enjoys some measure of stability compared to a period of soaring inflation years ago.

Tsvangirai said his Movement for Democratic Change party has in its possession evidence of massive rigging by Mugabe’s party in the just-ended polls and will challenge results from Wednesday’s voting in court.

“People of Zimbabwe must be given another chance to participate in a free, fair and credible election. They have been shortchanged by a predetermined election,” he said. He added that his party will not “participate in any government institutions” in protest but stopped short of saying it will boycott its reduced seats in the Harare parliament.

Mugabe’s loyalist army and police have set up security posts in Harare on Saturday, apparently in case there are any protest demonstrations.

“We are rejecting the results because they are fraudulent,” Tsvangirai said. “We will go back to our people. Our people are the ones hurting. Our people are disciplined. We don’t want a violent resolution to this crisis.”

He said a complete audit is needed of the shambolic lists of registered voters, which were made available to the parties only at the time of the election.

U.S. Secretary of State John Kerry said in a statement that the election process was “deeply flawed” because of irregularities in the voters’ roll, unequal access of the parties to state media, a lack of political reforms as mandated by the constitution, and other problems. He cited “the balance of evidence,” although the United States was barred from monitoring the vote.

“The people of Zimbabwe should be commended for rejecting violence and showing their commitment to the democratic process,” Kerry said. “But make no mistake: in light of substantial electoral irregularities reported by domestic and regional observers, the United States does not believe that the results announced today represent a credible expression of the will of the Zimbabwean people.”

British Foreign Secretary William Hague expressed “deep concern” over the election, saying the failure to share the voters’ roll with political parties was a “critical flaw.” Hague said that and other irregularities “call into serious question the credibility of the election.”

“We note that some political parties have rejected the result on the basis of these irregularities,” he said, without saying whether Britain endorsed their move. “We will need to examine what has happened and consider further reports from regional and local observer missions. In the meantime, it is important that all allegations of electoral violations are thoroughly investigated.”

Tsvangirai’s party lost several seats in its Harare urban strongholds by massive margins compared to their overwhelming victories in 2008.

In one Harare constituency, Tsvangirai’s party won with 9,538 votes to 8,190 captured by Mugabe’s party. In 2008, the same district voted for the MDC candidate, Tendai Biti, by about 8,300 ballots against Mugabe’s 2,500 votes.

Independent monitors have charged that as many as 750,000 voters were prevented from casting their ballots on Wednesday because of irregularities in voters’ lists. They also allege that thousands of unregistered voters were allowed to vote.

The continent-wide African Union and regional monitors of the Southern African Development Community, or SADC, have generally endorsed the elections as peaceful. But they have expressed misgivings over how voting numbers might have been manipulated and have demanded a full account of voter numbers from the official state election body before passing their final judgment on whether the polls were free, fair and credible.