Governement says Jawad Marhun was killed by sickle cell disease, but his advocates say otherwise [Reuters]

Bahraini security forces have fired tear gas, rubber bullets and stun grenades to disperse thousands of mourners at the funeral of a man who died after he himself was tear-gassed, a Shia politician said.

"Security forces used tear gas, rubber bullets and stun grenades to disperse the demonstrators while the majority of them were trying to leave at the end of the funeral," said Matar Matar, a senior member of Al-Wefaq, Bahrain's largest Shia opposition formation, on Friday.

Tens of thousands of Bahrainis, he said, were at the funeral of Jawad Marhun, a 36-year-old who died late on Wednesday from what Al-Wefaq said was "excessive exposure to tear gas from a canister tossed into his parents home on September 10".

The government of Bahrain, which crushed a month of protests in mid-March, said Marhun had died from "acute respiratory" problems as a result of sickle cell disease. His family denies that he had the disease, according to the Bahrain Centre for Human Rights (BCHR).

In April, the government said two other activists died from the blood disorder while in custody.

"Sooner or later, the people will achieve victory," the mourners chanted in the village of Sitra, according to Matar.

"He is the second victim within about two weeks in Sitra," said the former member of parliament.

"In general, the tension in Bahrain is increasing and the government is still using excessive force against the demonstrators and ignoring the demand for real dialogue that would lead to major political reforms."

The BCHR says that the government has intensified its use of tear gas in a campaign to stop unrest.

"We are receiving many complaints that the teargas being used now produces a black smoke rather than white, and is a lot stronger than the one security forces used to use," the BCHR said in a statement.

"The burning feeling in the chest is sharper, and the skin feels like it is burning (as described by protesters)."

On August 31, Ali Jawad Ahmed al-Sheikh, 14, died in the village of Sitra during a protest after prayers commemorating the beginning of the Muslim holiday of Eid al-Fitr.

The opposition said al-Sheikh was fatally wounded when he was struck in the face by a tear gas canister fired by security forces.

"According to those who took him to the hospital, those working at the hospital refused to treat the boy," said the BCHR.

The government said the teenager died of trauma after suffering a blow from a blunt object to the back of the neck that could not have been caused by a rubber bullet or tear gas.

The deaths come ahead of parliamentary elections scheduled for September 24.

The opposition has already boycotted the polls and wants democratic reforms in the Gulf kingdom where a Sunni monarchy has ruled over a majority Shia population for decades.

"I believe that the shortest path for stability can be reached if the king takes a brave decision to drive the kingdom (towards) tolerance and real political reform," Matar said.

Security forces were aided in the March crackdown on protesters after being backed by troops with tanks from neighbouring monarchies.

Authorities said 24 people were killed in the unrest, including four police officers. The opposition put the toll at 30.