MDC Secretary-General and Finance Minister Tendai Biti said police were examining whether foul play was involved in the accident. The party will conduct its own investigation, he said.
Prime Minister Morgan Tsvangirai should have been given proper security, Biti told reporters following a party meeting.
"If there had been a police escort, what happened would not have happened; the authorities could have avoided this omission," he said.
Tsvangirai’s wife, Susan, was killed when a truck veered into the opposite lane on Friday and slammed into their vehicle. She was thrown out of the car, which overturned and rolled three times, and was pronounced dead on arrival at hospital.
Tsvangirai suffered some head and neck wounds, but his condition was stable, Biti said.
"Mr. Tsvangirai is stable, but he’s in physical pain. The physical pain is dwarfed by the loss of his wife," he said.
The driver of the truck, which belongs to the United States Development Agency is in police custody.
President Robert Mugabe, who visited his old rival in hospital, said on Saturday the accident and the death of Tsvangirai’s wife was a tragic blow on a nation that was celebrating a new power-sharing government.
"We were celebrating this major development when the tragedy struck. It is very sad indeed," Mugabe said.
Tsvangirai and Mugabe formed a power-sharing government in February after months of talks to try to end a political and economic crisis that has brought Zimbabwe to ruin.
Several world leaders offered condolences to Tsvangirai, among them South African President Kgalema Motlanthe.
The crash occurred about 50 km (30 miles) south of Harare as Tsvangirai headed to his rural home in Buhera along the potholed Harare-Masvingo highway.
Tsvangirai, who turns 57 on Tuesday, had six children with Susan, 50. She was very popular among MDC supporters who would to chant "mother, mother" when she appeared at rallies.
She avoided the political spotlight but stood by Tsvangirai throughout his struggle as Mugabe’s most determined opponent.