A bomb hidden in a motor scooter exploded in a crowded vegetable market in Punjab state today, killing at least 31 people and wounding 70 others as Hindus celebrated a religious holiday, police said. The bombing, which police blamed on Sikh separatists, raised to 39 the number killed since Monday in the northern state. Sikhs campaigning for a separate nation have killed more than 500 people so far this year in Punjab, a rich northern farming state where they hold a slight majority over Hindus. Soon after the blast in Batala, 25 miles north of the Sikh holy city of Amritsar, Hindu mobs hurled stones at a police station 150 yards from the vegetable market, senior police superintendent S.K. Goel said. Goel was unsure whether any policemen were injured. The violence prompted officials to impose an indefinite curfew on the city, he said, adding that ``the situation is very tense.'' Most of those killed were Hindus and the wounded included both Sikhs and Hindus, said Sanjeev Gupta, another senior police official. Thirty-two of those wounded were in serious condition. Goel said the bomb exploded during a march by about 100,000 Hindus celebrating the birth of Rama, a heroic warrior-king. Rama is extolled in Hindu lore as an incarnation of Vishnu, a member of the supreme Hindu trinity of gods. Witnesses, contacted by telephone, said portions of bodies were hurled across the market, which was covered with blood-soaked pieces of vegetables. ``Some people were crying for help. Those who were lucky to survive ran away in panic. Many shops, scooters and one car were destroyed,'' said one witness who spoke on condition of anonymity. Earlier today, a bomb exploded on a bus in another town near Amritsar, killing the driver and one passenger, both Sikhs, senior police superintendent Anil Sharma said. Police blamed Sikh extremists for the blast near Taragarh, 17 miles northeast of Amritsar. Sikh Extremists initially targeted only Hindus in their campaign but now have also turned their guns on Sikhs who don't support the separatist cause. On Monday night, Sikh militants shot and killed four members of a Sikh family in another part of Punjab, police said. At Salghowal, 95 miles southeast of Amritsar, five or six Sikh gunmen burst into the farmhouse of a Sikh constable Monday night, killing the policeman, his parents and a younger brother, senior police superintendent Baramjit Singh said. Police also blamed Sikh extremists for an attack on a bus Monday night in Uttar Pradesh state, which is separated from Punjab by Haryana state, news agencies reported. United News of India said one police guard was killed and 11 passengers were wounded when five gunmen stopped the bus. But a second police guard killed one of the gunmen and the others fled, according to United News. Press Trust of India said police recovered a Chinese assault rifle bearing the inscription ``Khalistan Commando Force.'' Khalistan, which means ``Land of the Pure,'' is the name Sikh militants use for the independent Sikh nation they want to create in Punjab. The bus was halted while traveling between Bijnore and Muzaffarnagar, about 75 miles northeast of New Delhi and about 95 miles from the Punjab-Haryana state line. Most attacks by Sikh extremists have occurred in Punjab. Nationwide, Sikhs comprise only 2 percent of India's 880 million people, while Hindus account for 82 percent. The militants say members of the Sikh faith, which has roots in both Islam and Hinduism, are discriminated against politically and culturally by the Hindu majority.