Amnesty International has called for the immediate release of four journalists - including two Britons - being held by police in Turkey.
Vice News journalists Jake Hanrahan and Philip Pendlebury were among those apprehended in the Diyarbakir region, south-east Turkey, on Thursday.
It has not been confirmed why they were detained. Vice News said they were due in court on Monday on terror charges.
They had been filming clashes between police and Kurdish militants, it said.
Violent exchanges between security forces and youths from the outlawed Kurdistan Workers' Party (PKK) have erupted in the region in recent days.
The journalists' lawyer said their hotel rooms had been searched and their camera equipment and footage impounded by police, according to Amnesty International.
The human rights group's Turkey researcher Andrew Gardner said the trio should be released immediately unless the Turkish authorities could demonstrate credible evidence of criminal acts.
Vice News said the four journalists had been "detained without charge for three days".
A spokesman said it was working "vigorously with all relevant authorities to secure the safe release of our four colleagues".
The situation has also been condemned by PEN, which works to defend and promote freedom of expression.
Jo Glanville, director of English PEN, said it was an "unacceptable harassment of journalists who are fulfilling an essential role reporting on events of public and international interest".
Earlier reports by Reuters quoted security sources as saying the group were detained for reporting from the country's majority Kurdish south-east without government accreditation.
A Foreign Office spokesman said: "We are providing consular assistance and are in touch with the relevant authorities following the arrest of two British nationals in Diyarbakir."
Turkey, like a number of Western countries, considers the PKK a terrorist organisation.
A ceasefire in the long-running conflict with the group appeared to disintegrate in July, when Turkey began bombing PKK camps in northern Iraq.
Turkey has limited journalists to access to those region.
In January, Dutch freelance reporter Frederike Geerdink, who was based in Diyarbakir and had been covering Kurdish issues, was detained on suspicion of "propaganda for a terrorist organisation".