Hundreds of Bahrainis staged on Thursday a sit-in outside the offices of the United Nations demanding action over the "excessive" use by police of tear gas against protesters.

Protesters gathered outside the UN house in Manama, in a rally organised by the Shiite-led opposition, according to a statement by Al-Wefaq, the main opposition formation.

Police are regularly clashing with protesters who take to the streets in Shiite villages despite last year's brutal crackdown on a month-long protest that demanded democratic change in the Sunni-ruled Gulf monarchy.

Protesters, including men and women, wore medical masks and carried used tear gas canisters of the type reportedly used by police to disperse demonstrations, according to pictures provided by the opposition.

Dozens of used tear gas canisters were also piled up by the protesters outside UN offices, the pictures showed.

Protesters chanted slogans demanding the international community to "take a stance against those crimes, and dangerous violations by Bahraini authorities against unarmed" people, said the statement.

Representatives of the opposition delivered a letter addressed to UN Secretary General Ban Ki-Moon demanding action.

"This excessive use (of tear gas) is a daily flagrant violation to the UN's Basic Principles on the Use of Force and Firearms by Law Enforcement Officials," the letter said.

The UN High Commissioner for Human Rights last week criticised Bahraini forces for their "disproportionate use of force" as they sought to quell protests, saying their use of tear gas may have led to over 30 deaths.

"We have been receiving worrying reports of the disproportionate use of force by Bahraini security forces, including the excessive use of tear gas, the use of birdshot pellets and rubber bullets," said spokesman Rupert Colville.