"Shaikh Bakri was picked up by security forces as he was on his way to a local television station for an interview," a Lebanese security source said. He gave no reason for the preacher's detention.
Bakri, who is Syrian-born, left Britain on Saturday after the British government pledged to silence radical Islamists as part of measures taken after last month's bombings in London.
Bakri, who has lived in Britain for 20 years, used to live in Lebanon and holds Lebanese citizenship. He said on Tuesday he was merely taking a holiday and planned to return to Britain.
Associates had said earlier he would not come back.
British Prime Minister Tony Blair unveiled sweeping measures last Friday to silence or deport extremists, saying Britain would override human rights laws if necessary. He said Britain would ban two radical groups from operating in the country.
One was the British branch of Hizb ut-Tahrir, which Bakri was involved with. The other was a successor to al-Muhajiroun, to which he was closely linked, and which won notoriety for celebrating the 11 September 2001 attacks on the United States.
Bakri has said he is no longer involved with either group.