Mashaima was arrested Tuesday at Beirut airport based on an arrest warrant issued by Interpol, an official told French press agency AFP.
Lebanese authorities are seeking legal documents proving Mashaima had been pardoned, according to the official.
Mashaima, leader of the Haq movement, was among 25 men charged in Bahrain in October with forming an illegal organisation, engaging in and financing terrorism and spreading false and misleading information.
He and Said al-Shihabi, secretary general of the Freedom Islamic Movement, had been tried in absentia. Mashaima was in Britain on a trip for medical treatment when the charges were brought against the group. Shihabi is also staying in Britain.
King Hamad bin Issa al-Khalifa's pardon came in response to demands by anti-government protesters.
Thousands of protesters continued to camp in Manama’s Pearl Square on the Thursday, the 11th straight say of protests. Demonstrators overnight renamed the square "Martyrs' Roundabout", which they wrote and displayed on a Bahraini flag.
The opposition coalition said Wednesday that a call for dialogue by Crown Prince Sheikh Salman bin Hamad al-Khalifa lacked foundations.
The opposition has demanded the resignation of the government headed by King Hamad's uncle, Sheikh Khalifa bin Salman, as a condition for dialogue.