A British freelance journalist in Gaza will be held for 15 days, Hamas officials said Monday, in an unprecedented step against a foreign reporter since the Islamic militants seized control of Gaza in 2007.

Documentary filmmaker Paul Martin was detained Sunday at a Gaza courthouse where he was to testify on behalf of a defendant accused of collaborating with Israel, said Hamas Interior Ministry spokesman Ehab Ghussein.

Mr. Martin had just begun to speak when the prosecutor ordered police to arrest him, saying the Briton was wanted in the case, according to Ehab Jaber, the attorney for the Gaza man accused of collaborating. "The policeman put the handcuffs on him, and took him out of the court to prison. They were rough with him," said Mr. Jaber, who witnessed the scene.

Mr. Ghussein said Mr. Martin, who has produced reports in the past for British Broadcasting Corp. and the Times of London, is suspected of harming Gaza's security. He said the order to detain him for 15 days was based on a confession by a suspected collaborator with Israel—an apparent reference to the man on trial.

Mr. Martin has met with British consular officials since his arrest, Mr. Ghussein said.

The British Consulate in Jerusalem said Mr. Martin was 55 years old. A spokeswoman said the U.K. government was "very concerned" and has been in touch with Mr. Martin's family. She spoke on customary condition of anonymity.

Iyad Alami, a lawyer for the Palestinian Center for Human Rights in Gaza, met with Mr. Martin for half an hour on Monday. He was in good condition, Mr. Alami said, adding that he wanted to learn more about the case before deciding whether his group would represent the journalist. He wouldn't share any more details on the meeting.

The rights group's director, Raji Sourani, said earlier Monday that he was asked by Mr. Martin to represent him.

Mr. Martin's colleagues called for his immediate release. "We expect Hamas, as we do all parties, to respect the rights of every journalist on assignment to work without fear of being arrested," said the Foreign Press Association, which represents journalists covering Israel and the Palestinian territories.

Since Hamas wrested Gaza from Western-backed Palestinian President Mahmoud Abbas nearly three years ago, it has carefully avoided confrontations with foreign visitors, particularly journalists. It also has tried to reach out to the West in hopes of ending an Israeli-Egyptian border blockade.

Gaza's Foreign Ministry said Monday it "wishes to reassure all journalists working in the region that the Palestinian government guarantees their freedom to work in the Gaza Strip without interference."

Ahmed Yousef, a Hamas official, said Mr. Martin "is being detained for clear security reasons, and it is nothing to do with his job as a journalist or [him being] a Westerner."

The chain of events began Sunday when MR. Martin went to the military court to speak on behalf of Mohammed Abu Muailik, who is being accused of collaborating with Israel, said Mr. Jaber, Mr. Abu Muailik's defense attorney. The attorney said Mr. Martin had been working on a documentary about Mr. Abu Muailik, who has been in detention since June.

Mr. Jaber wouldn't discuss details of Mr. Abu Muailik's past, but said his client works in computer maintenance and has a business relationship with an Israeli partner. A spokesman for a Gaza militant group, the Abu Rish Brigades, said Mr. Abu Muailik is a former member. The brigades are a violent offshoot of Hamas rival Fatah, the movement led by Mr. Abbas.