Ethnic and criminal violence blamed on gangs has killed 65 people in Pakistan's financial capital of Karachi, with police the latest victims shot dead in a brazen ambush, officials said on Saturday.

The government has been left struggling for solutions to the worst wave of unrest to sweep the city in 16 years as extra deployments of police and paramilitary officers appear unable to stem the troubles.

Spiralling unrest is a major source of concern in Pakistan's biggest city, which is used by Nato to ship the bulk of its supplies to troops fighting in Afghanistan and which accounts for around a fifth of the country's GDP.

The violence has been linked to ethnic tensions between the Mohajirs, the Urdu-speaking majority represented by the Muttahida Qaumi Movement (MQM), and Pashtun migrants affiliated to the Awami National Party (ANP).

Gunmen ambushed police late on Friday, sparking gunbattles in which four officers were killed and more than 30 others wounded, officials said, bringing the death toll to 65 since Wednesday morning.

The police commandos, dressed in plain clothes, were targeted in the eastern neighbourhood of Korangi, which had previously been immune from the troubles.

"These policemen were in a van going on a raid on a tip-off when they were intercepted by armed men who started firing, injuring many policemen," senior police official Shaukat Hussain told AFP.

"The police returned fire and at least one attacker has been killed."

Television footage showed injured policemen being carried by their comrades and local residents into ambulances and private vehicles heading to hospital.

"Our hospital has received 32 injured policemen, four of whom are critically injured. They all have gunshot wounds," said Seemin Jamali, spokeswoman for the Jinnah Post Graduate Medical Centre.

Karachi city police chief Saud Mirza told AFP that four police were killed.

Speaking after the funerals of the dead policeman on Saturday, provincial police chief Wajid Durrani said two of the attackers who fired at the police van were arrested.

"We have caught two attackers and we are interrogating them about others," Durrani said, adding that 18 people who were kidnapped on Friday had been retrieved by police.

Provincial home minister Manzoor Wasan said he could not give details about which parties or ethnic groups were involved in the violence, but said that "some 100 suspects had been arrested so far".

Witnesses in Korangi said there were pockets of intense gunfire between armed groups with ordinary people too frightened to leave home. Dominated by Urdu speakers, the area also has Pashtun, Baluch and Sindhi populations.

Karachi, currently a city of 18 million inhabitants and the country's economic powerhouse, has seen its population explode since independence in 1947.

Its neighbourhoods have been swollen by a huge influx of migrants from across the country, but particularly the deprived Pashtun northwest, looking for jobs and more recently to escape Taliban and Al-Qaeda-linked violence.

Speaking off the record because they were not authorised to release the information to the media, two security officials confirmed that 65 people had now died in violence in Karachi since Wednesday morning.

The city's worst-affected areas are impoverished and heavily populated neighbourhoods where most of the criminal gangs are believed to be hiding.

Independent economist A.B. Shahid estimated that 20 percent of the city's business was shut down on Thursday with markets closed in southern neighbourhoods to protest against extortion money demanded by criminal gangs.

Underlining the brutality of the violence, one security official said bodies of those kidnapped and killed had been stuffed in sacks before being dumped in various parts of the city.

He said the bullet-riddled bodies of four young men who worked for a mobile phone company had been found in a van with their hands and feet trussed in the impoverished Shershah neighbourhood.

"At least 20 killed on Thursday were kidnapped and tortured by armed gangsters. Their bodies were later stuffed in sacks and thrown away in different areas," the security official told AFP on condition of anonymity.

Notes had been left inside the pockets of clothes worn by some of the victims that read "Want more bodies?", the official said.

The independent Human Rights Commission of Pakistan said 800 people have been killed in Karachi so far this year, compared with 748 in 2010.