Four US journalists have been arrested in Bahrain accused of illegally entering the country and being involved in illegal activities.
One of the reporters was identified as Anna Therese Day and the other three were said to be members of her crew.
Ms Day's family rejected the official allegations, and US officials said they could not comment.
The arrests happened on the fifth anniversary of an anti-government uprising which was violently put down.
Clashes broke out between protesters and police on Sunday as marches remembering the date were dispersed by security forces.
Bahrain's interior ministry said in a statement that the four were arrested in Sitra, a Shia-majority village south of the capital, Manama, which has repeatedly seen anti-government protests.
The four had entered the country between 11 and 12 February, it said.
"At least some of the arrestees were in the country as members of the international media but had not registered with the concerned authority and were involved in illegal activities," the statement said.
One of the detained was "wearing a mask and participating in attacks on police alongside other rioters in Sitra", the statement added. The other three were arrested at a security checkpoint in the same area.
The group Reporters Without Borders confirmed Ms Day's identity and called for the immediate release of the four, describing their detention as "inexcusable".
Ms Day's website says she is an award-winning journalist, with reporting experience in Bahrain, Israel, Libya, Syria and many other countries.
The names of the other three have not been revealed.
Bahrain has been wracked by political unrest since an uprising in 2011, with the kingdom's Shia majority demanding greater political rights from the Sunni-led government. Some violent attacks have blamed on militants linked to Iran.
The violent suppression of the protests left dozens killed, hundreds injured and many more arrested.