The head of Bahrain Centre for Human Rights has been arrested on arrival at Manama airport from Lebanon.
Nabeel Rajab was approached at the immigration area by two plain-clothed men and two police officers who produced an order from the public prosecutor for his arrest, one of his colleagues told Al Jazeera.
Bahrain's Interior Ministry said in a statement that an order had been issued for Rajab's arrest, but provided no additional details or specific reasons.
"They told him they have an order from the public prosecutor to arrest him," said Yousif Mahafda, from the Bahrain Centre for Human Rights.
"He asked why and they said they don't know."
Mahafda said he's quite worried about Rajab, who was able to call his wife to let her know that he was being taken to the police station on unspecified charges.
"They banned several human rights organisations three days ago, so I had a feeling that something was going to happen," said Mahafda.
Rajab was due in court on Sunday to face two charges in connection with organising and participating what the government says was an illegal protest in Manama.
The fact that the nature of the arrest warrant has not been revealed strikes other activists as cause for concern, given that Rajab had arranged his trip to Lebanon to ensure that he made it back in time for his court appearance.
"In fact, nobody knows [why Rajab has been arrested], we know they asked him to appear at the court on May 6," said Hussain Yousif, Middle East and North Africa coordinator of the Bahrain Press Association.
"So he returned to Bahrain on May 5."
Mahafda said Rajab arrived in Bahrain back from a human rights seminar in Beirut, where "he had been speaking to the press as usual."
Rajab, a key figure in drawing attention to human rights issues in the Gulf state since protests began last year, has been targeted for his activities.
He was questioned by police in January after a protest was dispersed and he told Al Jazeera that he was beaten by "riot police."
"They said, 'Are you Nabeel Rajab?' I said 'Yes', then they beat me more," Rajab told Al Jazeera after he was released from the Hospital.
The former head of the Bahrain Centre for Human Rights Abdulhadi al-Khawaja has been in prison since April 9, 2011. He has been on an intermittent hunger strike for around three months and is currently awaiting retrial, along with 20 other activists.
A possible reason for the arrest might be Rajab's recent appearance on a BBC news show, which drew sharp commentary from one government official on Twitter, said Mahafda.
"He threatened Nabeel Rajab, saying that they [the government] would not stay silent about this," said Mahafda, who himself has been arrested twice.
"They target everyone who works for Bahrain Centre for Human Rights. The Bahrain government has no respect for human rights."
Bahrain's King Hamad earlier this week accused the foreign media of exaggerating unrest and inciting violence in his country, saying that no one would tamper with the right of Bahraini citizens to "express their opinions".