Activists say regime forces attacked protesters with artillery fire and machineguns in the city on Monday, adding that soldiers are conducting house to house searches .
Thousands of Saudi demonstrators were mourning the 19-year-old Nasser al-Mahishi, who was killed by regime forces, when they were attacked. Mourners also chanted slogans against the US-backed Al Saud royal family.
Mahishi was shot by regime forces late on Sunday while walking down a street in Qatif with his friends. He succumbed to his wounds on Monday.
Similar anti-government protest also took place in the nearby town of Awamiyah, where protesters burned tires and blocked main roads. There were no reports of police violence there.
Tension has been high in the province after Riyadh decided to increase its troop presence in the region in a bid to quash anti-government protest there.
Saudi Arabia’s east has been the scene of anti-government protests over the past months with demonstrators demanding human rights reform, freedom of expression and the release of political prisoners.
Saudi protesters are regularly met with live rounds and tear gas, which has left dozens of demonstrators injured. Since the beginning of anti-government rallies in the region hundreds of activists have also been arrested.