A recently constructed road bridge collapsed in the southern Pakistan city of Karachi on Saturday, killing at least four people and injuring others, officials said.

At least two cars, a police van and a donkey cart were crushed under masonry. A truck with a trailer and a minibus lay upturned by the 100-meter (yard) -long, curved section of the overpass that crashed onto a road about 15 meters (yards) below. The cause of the accident was not immediately clear.

City mayor Mustafa Kamal said they were using heavy machinery and bulldozers to remove debris and pull out injured people trapped in at least two or three vehicles.

Azhar Faruqi, the city police chief, said they had transported two bodies to a hospital, while another two bodies could be seen inside a crushed car.

"Witnesses say they saw four or five vehicles on the bridge when it caved in," he said.

Hospital officials reported receiving at least five injured people.

Earlier, Mohammed Iqbal, a survivor who escaped through the window of a small bus that fell from the bridge, told the private Geo news channel that some other passengers were still trapped in vehicles and under masonry and he had heard their cries for help.

Prime Minister Shaukat Aziz expressed sorrow over the loss of life and concern over the quality of construction of the bridge -- a substantial concrete overpass in the west of Karachi, Pakistan's biggest city, which is increasingly plagued by traffic congestion.

The bridge was inaugurated by President Gen. Pervez Musharraf in August.

"Investigations have been ordered to find the reasons of the collapse and people responsible for substandard construction would be taken to task," Aziz said in a statement.

Bystanders and relatives of those trapped under the rubble were angered by slow rescue efforts and threw stones at police and paramilitary security forces deployed at the scene, injuring two. In response, the security forces baton-charged the crowd.

Rescue workers were using diggers to try and excavate one car which was crushed under masonry and had a woman's hand protruding from it. Authorities made announcements requesting the hundreds of onlookers to stay back amid fears that more of the overpass could collapse.