Four men have been killed in protests this week by the Shia minority in eastern Saudi Arabia in the most serious outbreak of violence in the Kingdom since the start of the Arab Spring.

The Saudi Interior Ministry said yesterday that two of those who died had been shot in an exchange of fire between police and gunmen who had "infiltrated" the funeral of another protester killed earlier in the week.

The latest shootings show that protests by members of the two-million-strong Shia community, mostly concentrated in the oil-rich Eastern Province, are escalating. The Interior Ministry says that "a number of security checkpoints and vehicles have since Monday been increasingly coming under gunfire attacks in the Qatif region by assailants motivated by foreign orders". It threatened harsh measures against anybody breaking the law.

Hamza al-Hassan, an opposition activist, said that the latest violence started last weekend when Nasser al-Mheishi, 19, was killed at a checkpoint near Qatif, an oasis which is a Shia centre. Mr Hassan says that "he was killed and left for three or four hours on the ground because the government refused to let his family collect the body". This led to mass protests in and around the city of Qatif in which a second man, Ali al-Feifel, 24, was shot dead by police, doctors were quoted as saying by news agencies. The Saudi Interior Ministry could not be reached for comment on the allegations.

The Shias in Saudi Arabia have long complained of political, social and economic discrimination against them by the state. Demonstrators earlier this year demanded the release of nine Shia activists held without trial since 1996.

The latest round of fighting took place on Wednesday during a funeral for one of the protesters. "Security forces have been exercising self-restraint as much as possible," said the official statement, which did not specify who fired the fatal shots. Six people were wounded including two policemen who received gunshot wounds, it added.

During riots around a police station in nearby Awamiya town in October, the ministry claimed that police used only rubber bullets and live fire came only from demonstrators.

The Saudi government asserts that the latest confrontations by protesters were "ordered by their masters abroad" – by which the government invariably means Iran.

Mr Hassan said that local Shia leaders had been to see the governor of the province, Prince Mohammed bin Fahd, in the provincial capital, Dammam, who had told them that the government's patience was at an end and asked them to calm their people. Another Shia leader, Hussain al-Biat, reportedly said to Prince Mohammed: "Don't shed more blood or you will lose control."