Police said Raja Muhammad Nasrullah, former president of the Tehrik-i-Jafria Pakistan party, was killed when a grenade was thrown at his home in Punjab province by unidentified attackers.

News of his death followed two attacks on Sunni Muslims earlier on Saturday, which were themselves thought to be revenge killings.

At least 25 people, most of them belonging to the Shi'a community, have died since Friday.

The BBC Islamabad Correspondent Owen Bennett-Jones says it appears to be a co-ordinated campaign, with gunmen firing automatic weapons from motorcycles and cars.

Pakistani police said they had detained about 500 people in a crackdown on suspected religious militants.

Members of the Shi'a community have accused a hardline Sunni group, the Sipah-e-Sahaba, of starting the latest cycle of violence, but the group has denied this.

Iran - which is mainly Shi'a - has urged the rival Muslim groups to resolve their differences through dialogue.

Attacks in Karachi

At least five Sunni Muslims were killed in two separate incidents in the city of Karachi on Saturday.

In one attack, a white car drew up outside a Sunni Muslim religious school and one of the passengers opened fire on students in the street outside.

At least three died and around a dozen were injured, some seriously.

Two other people died when gunmen opened fire on a shop. They were members of a hardline Sunni party.

The authorities have condemned the killings and various official inquiries and investigations have been announced.

In the past, sectarian violence in Pakistan has tended to be restricted to the province of Punjab.

The authorities are likely to be especially concerned that so many of the recent killings have taken place in Karachi - the country's main commercial city.

There have also been killings in Moltan, Peshawar and Lahore.

Relative calm broken

Sectarian violence involving extremist groups within the rival Sunni and Shi'a communities has been a fact of life in Pakistan for many years.

However there was a period of relative calm earlier this year following a security operation which saw most of the militants either arrested or forced into hiding.

But the shootings of the last few days have completely unnerved the authorities.

Senior police officials say armed sectarian groups have never before acted in such an organised way to simultaneously attack rivals in different cities.