The truck driver told the police that he had fallen asleep at the wheel, Mr. Murira said.

The crash, coming less than a month after Mr. Tsvangirai was sworn in as prime minister in a tense and long-negotiated power-sharing government with his rival, President Robert Mugabe, stirred deep suspicions in his party, but most officials were careful to say not enough was known about the collision to make any accusations of foul play.

Mr. Mugabe and his wife paid a condolence call to Mr. Tsvangirai at the hospital on Friday evening.

Ian Makone, a secretary in the prime minister’s office, said he arrived at the crash scene about a half hour after the fact. He said one of the drivers in Mr. Tsvangirai’s convoy told him that an oncoming truck “had clipped the right rear fender of Morgan’s car.”

Mr. Murira said the prime minister told him the driver of Mr. Tsvangirai’s vehicle swerved to avoid the on-rushing truck, but a trailer attached to the truck hit the Land Cruiser, which rolled over three times. Mr. Makone said the vehicle was lying on its roof when he arrived.

Eddie Cross, the policy coordinator for the Movement for Democratic Change, said that when he heard about the crash he phoned Hendrick O’Neill, a party member who is from the area where it occurred. Mr. O’Neill, in turn, contacted Deon Theron, the vice president of the Commercial Farmers Union, who lives near the scene.

“I was looking for someone to get to the site because I was very suspicious about the circumstances around the accident,” Mr. O’Neill said in a phone interview. “Morgan has been a target for some time.”

Mr. Theron rushed to the scene and began to investigate, Mr. O’Neill said: “Just as he finished the police arrived and grabbed the video camera from him, started questioning him and took him into custody.”

Mr. O’Neill said he spoke to Mr. Theron by cell phone as he was being arrested. “He told me the left front tire had burst and the vehicle was on its roof,” Mr. O’Neill said. “He climbed on the vehicle. Some of the undercarriage was loose or broken. It could have been the result of the accident. That’s what he was filming when they seized him.”

Mr. Tsvangirai has been the victim of multiple assassination attempts. He fled the country after he outpolled Mr. Mugabe in March presidential elections, fearing for his life. Forces loyal to Mr. Mugabe had begun a campaign of violence, attempting to intimidate the opposition prior to a June runoff election.

Mr. Tsvangirai ended up withdrawing before that second poll because of attacks on thousands of his supporters. When the international community concluded the election was neither free nor fair, protracted negotiations led to a coalition government, with Mr. Mugabe as president and Mr. Tsvangirai as prime minister.

Friday night, officials with Mr. Tsvangirai’s party alternately expressed their suspicions of foul play but resisted reaching any conclusions. “This will certainly demand an independent investigation,” said Mr. Cross. “We won’t accept a police report.”

The couple, married for more than three decades, has six children, including twins, aged 14.

“They were a team; they were very effective and extremely close,” Mr. Cross said of the couple. “She was very much a pillar of support, spiritually and in every other way. Morgan will feel her loss enormously. I can’t think of many couples as close as those two.”