(Updates with killing of "U.S. spy") TANK, Pakistan, March 9 (Reuters) - Gunmen on a motorcycle shot dead a Shi'ite Muslim activist in northwestern Pakistan on Friday, a day after a member of a Sunni Muslim militant group was killed in a similar attack, police said. Pakistan's minority Shi'ite community is observing a major religious commemoration on Friday and Saturday, and police in the south of the country said they had arrested a wanted Sunni militant planning an attack on a Shi'ite procession. Pakistan has been bedevilled by sectarian violence for years. Thousands have been killed in tit-for-tat attacks by Sunni and Shi'ite militants since the 1980s. No one claimed responsibility for killing Anwar Abbas Shah as he sat in a shop in the town of Dera Ismail Khan, 270 km (168 miles) southwest of Islamabad. Shah ran a community centre and police said they suspected it was a sectarian attack. "He died on the spot while a man working for him was wounded," said senior police officer Aslam Khattak. "It appears to be a sectarian attack but we're investigating." On Thursday, gunmen killed a member of a banned Sunni militant group in the town. In a separate incident, suspected Pakistani Taliban militants shot dead an Afghan refugee in the North Waziristan tribal region after accusing him of spying for the United States, residents said. His body was found dumped on Friday. Militants in Waziristan have killed dozens of people suspected of supporting the Pakistani government or spying for the United States, which has about 27,000 troops in Afghanistan. Shi'ite Muslims commemorate Chelum, or Arbain, on Friday and Saturday, a ceremony marking the end of a 40-day period of mourning for the Prophet Mohammad's grandson who died in AD 680. Police in Pakistan's southern province of Sindh said they had arrested Sunni militant suspect Jaleel Ababaki, who they said had been planning an attack on a Chelum procession in the town of Sukkur on Saturday. Sukkur police officer Mazhar Nawaz said Ababaki belonged to the Lashkar-e-Jhangvi militant group and was wanted for involvement in a 2004 attack on Shi'ites in the southwestern city of Quetta in which 44 people were killed. Nawaz said Ababaki had been found with bomb-making material. Several other Lashkar-e-Jhangvi members have been arrested in recent weeks on suspicion of planning attacks on Shi'ites. Shi'ites make up about 15 percent of the Sunni-majority country's population. Separately, police in the northwestern town of Tank arrested two men with explosives on Friday after a tip-off from a suspected suicide bomber arrested in the town on Thursday.
