LONDON—Police said they arrested nine men Thursday on suspicion of supporting terrorism and belonging to a banned organization as part of a continuing investigation into Islamist-related terrorism.

U.K. media, including the British Broadcasting Corp., reported that the suspects included one of Britain's most high-profile, radical Muslim preachers, Anjem Choudary, and others connected to his now-disbanded al-Muhajiroun group.

London police didn't release the names of the suspects and wouldn't comment on the media reports. Mr. Choudary's telephone appeared to have been disconnected and it wasn't clear whether he had retained a lawyer.

Though Mr. Choudary has voiced support in interviews and online for Islamic State, he has said that he has never condoned violence or terrorism.

Mr. Choudary, a lawyer who was a frequent guest on British television and radio, espouses the primacy of Islamic law, known as Shariah. He also says that Western societies are unclean because they are not based on Islamic precepts.

He has been a vocal critic of British foreign policy and Western intervention in Syria, Iraq and other Muslim countries. He and his group's members have formed multiple successor organizations since Muhajiroun was shut by the government in 2010 under antiterrorism legislation passed that year.

London police said that the arrests were part of a continuing inquiry into Islamist-related terrorism and not in response to any immediate threat. They were also searching 18 properties in London and one in Stoke-on-Trent in central England.

The arrests came as Prime Minister David Cameron prepared to hold a parliamentary debate Friday on whether the U.K. should participate in airstrikes against Islamic State in Iraq, amid gathering domestic support of military intervention.

Late last month, the U.K. raised its assessment of the level of the threat from international terrorism to severe, the second-highest on its five-point scale, citing gains for the Islamist militants in Syria and Iraq.

Mr. Choudary's radical views have alienated him from most mainstream Muslim groups in Britain. In recent years, he has courted controversy by protesting British soldiers who have served in Afghanistan and Iraq, and for his association with several British citizens who were later convicted on terrorism charges. They include Michael Adebolajo, one of the killers of Lee Rigby, a British soldier who was attacked on the streets of London.

In a 2013 interview with The Wall Street Journal, Mr. Choudary said he knew Mr. Adebolajo under his nickname Mujahid. But he said he had no inkling that Mr. Adebolajo, who he said had converted in 2003 and wasn't a member of his group, would carry out a terrorist attack.

A British antiracism activist group published a report this year that concluded that at least 70 British Muslims who have been convicted of terrorist crimes in Britain or committed suicide attacks abroad had links to Mr. Choudary or his Muhajiroun followers.

Over the last two weeks, Mr. Choudary was scheduled to give two series of lectures via an online chat room that is popular with jihadists and people who espouse the puritanical strain of Islam known as Salafism. The topics of the lectures were about the barriers to Muslims living in Western societies and his criticisms of the British legal system.

In 2006, a law came into effect that made it illegal to praise, celebrate or glorify terrorism. The law also made it illegal to disseminate terrorist publications and was part of an effort to tighten up laws on who can preach to British congregations.

Write to Margaret Coker at margaret.coker@wsj.com and Jenny Gross at jenny.gross@wsj.com