HARARE, Zimbabwe — The ruling Zanu-PF party lost its majority for the first time in 28 years to the opposition Movement for Democratic Change.

The MDC claimed that it had won the separate presidential poll, with its leader, Morgan Tsvangirai, taking 50.3% of the vote to President Mugabe’s 43.8%.

But the government refused to release the official result and suggested that neither man had won the 50% necessary to avoid a second round in three weeks. If Mr. Mugabe insists on the runoff, he faces an embarrassing defeat.

Aides of Simba Makoni, the third presidential candidate who took 7% of the vote, said: “We are going to side with Tsvangirai.”

The MDC secretary general, Tendai Biti, said: “Mr. Tsvangirai has won this election. He is the next president of the republic of Zimbabwe, without a run-off.” He said the party would accept a second round “under protest,” in the expectation of victory.

“It is unlikely that the will of the people will in any way be reversed in that run-off,” he said. “If anything, there will actually be an embarrassing margin in favor of the opposition.”

The official Herald newspaper, a mouthpiece for the Mugabe regime, predicted that neither leader had scored 50%. The front-page report is the clearest possible indication that Mr. Mugabe is determined to keep fighting.

In return for his support, Mr. Makoni would be seeking the deputy presidency or prime ministership in a government of national unity led by Mr. Tsvangirai. He would also want undertakings on a new constitution and fresh elections, aides said.

Official parliamentary results yesterday showed that Zanu-PF had lost its majority for the first time since independence in 1980. The two factions of the MDC won 105 seats in the 210-member house of assembly, with Zanu-PF on 93 and one independent.

The regime’s willingness to concede a second round in the presidential election indicates that declaring first-round victory had become politically impossible. But analysts warned that Mr. Mugabe could fight to the last.

There is speculation that a huge rigging operation will be attempted after a ruling party spokesman said Mr. Makoni’s supporters would flock back to Zanu-PF in a second round of voting. Mr. Mugabe can still call on an army of so-called war veterans of the independence struggle and the feared youth brigade if he decides on a violent final last stand.

“He is not the type that quietly walks away into the sunset,” a Western diplomat said.

There were fears in Harare last night that a hardline Zanu-PF faction would mount a violent purge of the party to ensure it had “undiluted” capacity to eliminate Mr. Tsvangirai, with widespread violence used to prevent the MDC from taking power.