Pakistan's largest city Karachi shut down on Monday to mourn 45 people killed by a car bomb in a Shiite Muslim neighbourhood, the latest in a series of devastating attacks ahead of general elections.

Traffic was thin as educational institutions, businesses and markets closed after the local government announced one day of mourning and Shiite groups three days of mourning for those killed in Sunday's bombing in Abbas Town.

The bomb exploded as worshippers were coming out of mosques, ripping through two apartment blocks, setting one of them on fire and trapping people beneath piles of rubble. Survivors were on Monday being housed temporarily in schools.

There has been no immediate claim of responsibility. But suspicion will likely fall on banned Sunni extremist organisation Lashkar-e-Jhangvi, which has claimed major attacks on Shiites in the city of Quetta, and on the Pakistani Taliban.

Rights groups and ordinary citizens have strongly criticised the government for failing to stop sectarian murders and bombings and bring to justice those responsible.

"Terrorists are killing us but the government is not taking any action to eliminate them," said Mohsin Ali, 29, a Shiite whose elder brother was killed.

"How long will we keep losing our children, our relatives?"

Survivors could be seen searching for personal items and belongings such as jewellery from the rubble of their apartments.

"The government should provide us with arms to deal with terrorists if their agencies are unable to bring them to book," said Azam Khan, a Sunni Muslim, who said he had taken several of the dead to hospital.

"We will vote for those who eliminate these terrorists. We are not ready to be hoodwinked by empty slogans any more."

Karachi is vital to Pakistan's economy, contributing 42 percent of GDP, 70 percent of income tax revenue and 62 percent of sales tax revenue.

But the city is plagued by sectarian, ethnic and political violence which last year killed more than 2,200 people and which routinely forces closures.

On Monday the stock market was open, but Mohammad Sohail of brokerage firm Topline Securities said there were fewer dealers and trading volumes were low.

Local transport and traders' associations stopped work.

Officials said 45 people, including women and children, were killed and around 150 wounded in the attack. Although Abbas Town is a mainly Shiite neighbourhood, officials said some Sunnis were among the dead.

Pakistan's parliament is due to dissolve in two weeks in preparation for elections. But rising violence against Shiites, who make up around 20 percent of the 180 million population, has raised serious questions about security.

Last year was the deadliest on record for Shiites in Pakistan with more than 400 killed, according to Human Rights Watch.

Last Tuesday the Supreme Court ordered the authorities to come up with a strategy to protect Shiites after bomb attacks on Shiite Hazaras in the southwest on January 10 and February 16 killed nearly 200 people.

Police on February 22 detained Lashkar-e-Jhangvi leader Malik Ishaq for 30 days under a law intended to maintain public order.

He was detained briefly in 2012 for inciting sectarian hatred, having been released on bail in 2011 despite being implicated in dozens of murders.

The Pakistani Taliban have also increased attacks in recent months, leading to fears they could disrupt the election scheduled to take place by mid-May.

Last month the group proposed talks with Islamabad. But the government insists the militants must declare a ceasefire before coming to the negotiating table -- a condition they have rejected.