CHANDIGARH (Reuters) - Troops patrolled towns and highways in Indias northern state of Punjab to enforce a curfew on Tuesday after three people were killed in riots sparked by the killing of a Sikh preacher in Austria. More than 18,000 bus and railway passengers had been stranded in Occupied Kashmir after the violence brought some services to a standstill, but the routes started to resume traffic late on Tuesday afternoon. Thousands of dalits torched trains, cars and shops on Monday to protest against the killing of a leader of a religious sect in an attack on a temple in Vienna, prompting Indian Prime Minister Manmohan Singh to appeal for calm. The preacher who died was from the Dera Sach Khand, which has a large dalit following in India of lower caste Sikhs and Hindus. Most shops in parts of Punjab hit by Mondays protests remained closed for a second day, but authorities began to slowly relax the curfew in some areas. The curfew has been relaxed from 10 am to 2 pm and the time could be extended if there is no violence, senior state police official Vikas Garg said. At least 16 people were hurt in the violence in Vienna on Sunday when six armed men attacked two preachers visiting from India during a temple ceremony. Austrian police said they did not know yet about the motives for the attack. Sikhism officially rejects caste but social hierarchies still prevail. Dera Sach Khand has gained many followers from the dalit community in Punjab and is opposed by many mainstream Sikhs.