Iran's biggest Shia religious holiday was overshadowed by a suicide bombing today that killed 41 people in eastern province of Sistan-Baluchistan. It was claimed by a Sunni rebel group.

The attack at the Imam Hussein mosque in the port city of Chabahar, close to the Pakistani border, happened during a religious ceremony on the eve of Ashoura.

More than 50 people were injured in the attack involving four bombers, two of whom detonated explosives attached to their belts. A third was shot at by Iran's intelligence service and the fourth was arrested, the Fars news agency said.

Iran's deputy minister of interior for security affairs, Ali Abdollahi, said there was evidence the bombers were "supported by regional and intelligence services". State-run Press TV reported that the explosive belts were obtained from Saudi Arabia. The US and Britain condemned the attack. "This is a disgraceful and cowardly act," said President Barack Obama.

Jundollah, the group that claimed responsibility, has a long history of targeting civilians. Often said by Tehran to be secretly supported by the US, Britain or Israel, it reportedly uses bases in Pakistan to mount operations in Sistan-Baluchistan, which has a Sunni majority.

Its timing during the Shia festival marking the 7th-century martyrdom of the Prophet Muhammad's grandson Hussein suggests a clear sectarian motive.

Jundollah's last big bomb attack was on Hussein's birthday in July on a mosque in the provincial capital of Zahedan. That was described as retaliation for the execution of the group's captured leader, Abdolmalek Rigi, and killed 28 people, including members of the Revolutionary Guards.

Ashoura is one of the high points of the Shia calendar when crowds of worshippers gather in mosques across Iran. The government had warned against protests by the opposition Green movement, still seeking to galvanise support over the disputed presidential election in 2009.

During Ashoura that year there were violent clashes between security forces and protesters.

Opposition supporters have complained in recent days of a new crackdown involving the intimidation, harassment and arrest of journalists, civil rights activists, students and their families.

Last week the Shargh newspaper, one of the country's few independent papers, was raided. Seven journalists were arrested on Student Day, which commemorates the deaths of protesters in 1953.

Iranians at home and abroad say they are puzzled by the intense focus in some western media on the case of Sakineh Mohammadi Ashtiani, the woman sentenced to death by stoning for adultery, when so many people are now affected by routine state repression.

More than 100 journalists and bloggers have been jailed in Iran since the disputed election, making it the world's leading enemy of free expression. At least 65 remain in jail.

Relatives of three people killed last year, Shabnam Sohrabi, Shahram Farajzadeh and Mostafa Karimbeygi, told Roozonline, an opposition website, that they have intimidated and harassed.

The website of the Green movement leader, Mir Hossein Mousavi, reported today that Mohammad Nourizad, a documentary film-maker, was suffering from a stomach ulcer because of a hunger strike in prison.

Last week the parents of protesters killed in the 2009 protests were arrested in Behesht-e-Zahra cemetery, Tehran, while celebrating the birthday of Amir Tajmir, who is buried there. The mother of Sohrab Arabi and the father of Ramin Ramezanifar, who also died in the unrest, were detained. Arabi's mother, Parvin Fahimi, has become politically active since the death of her son and has repeatedly spoken out in support of those in prison.

Yesterday the authorities released Kouhyar Goudarzi, a journalist and human rights activist initially charged with crimes against god (Moharebeh) – punishable by death – after one year in prison. Sara Sabbaghian and Maryam Kian Arasi, both human rights lawyers, were also freed.