Media playback is unsupported on your device Media caption The BBC's Mike Wooldridge said the attack would be "deeply worrying for the government"

At least 45 people have been killed by a bomb explosion in the Pakistani city of Karachi, police say.

The blast in the mainly Shia Muslim area of Abbas Town destroyed several buildings and set others on fire. Some reports spoke of a second explosion.

No group has yet said it planted the bomb, which went off near a mosque as worshippers left evening prayers.

Pakistan's Shia minority are the target of frequent sectarian attacks from Sunni militant groups.

Analysis Shia Muslims make up about 20% of Pakistan's population of 180 million, and while sectarian violence targeting them is by no means a new phenomenon it is on the rise. Human Rights Watch says that last year more than 400 Shias were killed in such attacks across the country. Just in the first two months of this year, nearly 200 Shias - members of the Hazara community - have been killed in the south-western city of Quetta. Karachi, where this latest anti-Shia bombing has taken place, is notorious for violence of various kinds. But it is clear that the ability of the authorities to protect Shias is now being tested even more severely - and this as new elections are around the corner. Political and religious leaders were quick to condemn the Karachi attack and promise help to the victims.

The explosion sent a huge column of smoke into the sky above Karachi and caused a power cut in part of the city.

Police are investigating whether it was a suicide attack.

Rescuers have been struggling to reach people trapped under the rubble.

Residents have been using car headlights to help the search for survivors, local media reports said.

Around 150 people were wounded by the explosion, officials said.

"I was watching television when I heard an explosion and my flat was badly shaken," Karachi resident Mariam Bibi told Reuters news agency.

"I saw people burning to death and crying with pain. I saw children lying in pools of their own blood and women running around shouting for their children and loved ones." she added.

Rescue work was delayed as some residents fired guns into the air in anger at the carnage, reports say.

Sectarian divide

Pakistan's main political and religious leaders rushed to condemn the attack - the latest to target the Shia minority.

Nearly 200 people were killed in two separate bombings targeting the Shia community in the south-western city of Quetta in January and February.

Some relatives of the victims there initially refused to bury their dead in protest at what they said was the failure of the authorities to protect their community from attack.

No group has yet admitted to carrying out the Karachi bombing, but correspondents say suspicion is likely to fall on Sunni militant groups.

Sunni and Shia Muslims Muslims are split into two main branches, the Sunnis and Shias

The split originates in a dispute soon after the death of the Prophet Muhammad over who should lead the Muslim community

There are also differences in doctrine, ritual, law, theology and religious organisation

The great majority of Muslims are Sunnis

Pakistan - where Shias are a minority - has a history of sectarian bloodshed dating back to the 1980s Formidable power anti-Shia militants

Groups such as Lashkar-e-Jhangvi regard Shia Muslims as heretics and have stepped up attacks in recent years.

They are thought to have set several training camps for militants and police seizures have shown they have access to large quantities of weapons and explosives, the BBC's M Ilyas Khan in Islamabad says.

Some activists called 2012 the worst year in living memory for attacks on Pakistan's Shia community.

But already this year bombings in the south-western of Quetta alone have killed nearly 200 people.

Last month Pakistan's Supreme Court called on the authorities to devise a strategy to protect Shia Muslims more effectively, given the increase in attacks.

Karachi - Pakistan's biggest city and commercial capital - has a long history of violence.

As well as a sectarian divide between Sunni and Shia, that city has also seen conflict between different ethnic communities - Pashtuns from north-west Pakistan, Mohajirs (immigrants from India following the Partition) and Sindhis.