DERA ISMAIL KHAN, Pakistan, Dec 24 (Reuters) Gunmen opened fire on a Sunni Muslim funeral in Pakistan today, killing four people in what appeared to be a sectarian revenge attack.

The attack in the northwestern town of Dera Ismail Khan came a day after a Shi'ite Muslim professor was shot dead. His killing sparked violence protests by minority Shi'ites who ransacked some shops and blocked roads with burning tyres.

''Mourners were returning from the professor's funeral when armed men opened fire on the funeral of a Sunni Muslim that was taking place nearby,'' senior police officer Hammad Abid told Reuters.

Four people were killed and eight wounded, he said.

Four gunmen had opened fire on the funeral from a graveyard.

Two of the attackers were arrested, Abid said.

Sunni protesters then took to the streets attacking some shops. Soldiers were called in to guard a hospital in the area, Abid and witnesses said.

Shi'ites make up about 15 per cent of Sunni Muslim-majority Pakistan's 160 million people.

Thousands of people have been killed in tit-for-tat killings by sectarian militants in Pakistan over the past two decades.

REUTERS DKA RK1958