Osanlu's arrest takes place just three months after he was freed from Tehran's notorious Evin Prison on a large bail (about $165,000).
Some observers said that the Iranian government crackdown against workers and union members might strengthen their resolve to fight for their fundamental rights.
Returned To Jail
Osanlu was detained by security agents and reportedly taken back to Evin Prison, where he had been jailed for more than seven months.
Ebrahim Madadi, the deputy director of the Syndicate Workers of the Tehran Bus Company, was with Osanlu at the time of his arrest.
"On Sunday at 9 a.m., Mr. Osanlu, Mr. Mansur Hayat Gheybi -- a senior member of the syndicate -- and I were going to the labor office to follow up on the case of syndicate workers who have been fired from their jobs," Madadi said. "On the way Mr. Osanlu got out of the car to buy some newspapers, and while crossing the street he was arrested by agents from the Intelligence Ministry."
Madadi told Radio Farda that some seven-security agents dragged Osanlu toward their cars and took him away. He said they refused to show any identification or provide an arrest warrant.
Forcibly Detained
"Mr. Onsalu called for help while he was being arrested," he said. "I went to help him but the agents beat me, too. One of them who beat me pulled out his gun and fired a shot in the air. Currently we have no news about Osanlu's fate."
Madadi is concern about Osanlu's health and says he had an eye operation a few days before his arrest. His eye was bandaged when he was arrested.
He and other members of the Syndicate of Workers of the Tehran Bus Company have condemned the arrest of Osanlu as a violation of his rights and called for his immediate release.
Osanlu was due to appear in court today to answer the charges that have been brought against him. His family and lawyers have said that authorities have not informed them of the reason for Osanlu's rearrest or about any criminal charges made against him.
Osanlu was arrested in December after organizing protests against the working conditions of bus drivers.
His arrest was condemned by activists in Iran and also international groups, including the International Confederation of Free Trade Unions (ICFTU) and the International Transport Workers' Federation (ITF).
In July, dozens of rights activists in Iran and elsewhere went on a three-day hunger strike to call for his unconditional release as well as the release of other Iranian citizens who are held because of their peaceful activities and opinions.
Harassing Unions
Osanlu's case was highlighted as an example of growing state pressure on union activists and workers in Iran.
Dozens of workers who had participated in protest actions have been arrested on unspecified charges in recent months. Many were detained in January ahead of a planned strike by Tehran bus drivers that was organized to protest the detention of Osanlu and the recognition of trade union activities.
The ICFTU and the ITF have said that they will continue to raise the plight of Iranian bus union workers and apply pressure on Iranian authorities to convince them to respect worker's rights.
Both organizations launched a formal complaint in July against Iran to the International Labor Organization to protest the detention of Osanlu and the crackdown against his group.
Some observers said that the Iranian government crackdown against workers and union members might strengthen their resolve to fight for their fundamental rights.
Just two weeks ago, workers from a closed down factory in Iran's northern city of Rasht threatened to protest the closure by appearing in front of the presidential palace. They said they would walk barefoot from Rasht to Tehran while wearing special white sheets used for burials (kafan).
(Radio Farda Deputy Director Hamid Fatemi contributed to this report.)