HARARE, Zimbabwe — Zimbabwe’s opposition heckled President Mugabe in an unprecedented show of defiance when the president opened parliament today with traditional pomp and his familiar denunciations of the West.

Mr. Mugabe arrived in an open-topped vintage Rolls Royce escorted by mounted police wearing pith helmets and carrying lances. Legislators from the Movement for Democratic Change, who wrested control of the house from him for the first time since independence in 1980, refused to stand when Mr. Mugabe entered, and shouted his party “is rotten!”

The jeers occasionally drowned out his 30-minute speech that was broadcast live on national television. Mr. Mugabe had to raise his voice and, looking annoyed, raced through the final lines.

Today’s tension may be a glimpse into a future of bitter debates and close votes once parliament gets down to work in October.

Mr. Mugabe’s ZANU-PF party had held a parliamentary majority since 1980. Morgan Tsvangirai’s Movement for Democratic Change won 100 seats in the 210-seat legislature in March elections — though only 99 of its members were able to take the oath of office yesterday, after one was arrested as he tried to enter parliament.

Mr. Mugabe’s party won 99 seats in March and a splinter opposition faction won 10. An independent who broke away from Mr. Mugabe’s party has the remaining seat.

In parliament yesterday, Lovemore Moyo of the Movement for Democratic Change won the race for speaker by 110 votes to 98. The ballot was secret, but Mr. Moyo apparently got votes from both Mr. Mugabe’s party and the splinter faction to win a post that puts him in charge of parliament’s debate and schedule and gives him the power to appoint committee chairmen.

If the opposition continues to win support from the splinter faction, it would have the simple majority needed to block funds for government ministries and projects. But if there was deadlock, Mr. Mugabe could dissolve the assembly and rule by decree. And it was unlikely the opposition could summon the two-thirds vote needed to impeach Mr. Mugabe.

Mr. Mugabe opened parliament despite an agreement he signed last month with Mr. Tsvangirai that the assembly would not sit unless both men agreed or until a power-sharing deal was struck.

Negotiations, though, have deadlocked over whether Mr. Tsvangirai or Mr. Mugabe would have the leading role in a unity government.

Mr. Tsvangirai beat Mr. Mugabe and two other candidates in presidential elections held alongside the legislative balloting, but did not gain the simple majority needed to avoid a runoff. Mr. Mugabe held a one-man runoff after allegedly unleashing his soldiers, police and party militants on the opposition, and declared himself victor despite international condemnation.

The opposition blames Zimbabwe’s crisis on Mr. Mugabe’s increasingly autocratic rule and economic mismanagement. Mr. Mugabe ordered the seizure, at times violent of white-owned commercial farmland, saying it would be turned over to blacks, but in many cases handing farms to cronies, in the process destroying the country’s economic base.

Mr. Mugabe has repeatedly blamed his country’s woes on America and former colonial power Britain. He returned to that theme today, calling Western sanctions illegal.

“Sanctions must go,” he said, to cheers from his supporters. “They cannot last a day longer if we as Zimbabweans speak against them in deafening unison.”

Sanctions imposed by America and the European Union affect individuals and companies linked to Mr. Mugabe, and include travel bans and asset freezes.

While such targeted sanctions are meant to spare ordinary Zimbabweans, already suffering amid their economy’s collapse, Zimbabwean officials say they contribute to a climate that discourages foreign investment, loans and aid.

Mr. Mugabe today also accused Britain and America of unleashing “a vicious onslaught” against Zimbabwe. He said Zimbabwe, once the region’s breadbasket, was importing food from its neighbors, but that prices were increasing.

“Regrettably we have noticed the hand of our enemies to thwart us,” he said. “Food is the latest weapon in their regime change agenda.”