Muhammad al-Maghrabi, 64, was imprisoned after he continued to practice law despite being disbarred in absentia for allegedly bringing the judiciary into disrepute.
Al-Maghrabi rejects the Beirut Bar Association's charge that he was illegally practising law while disbarred.
"I am entitled to continue to practise until a court of law, such as the Court of Appeal... decides otherwise," he wrote in one of his recent letters from prison.
But he also suggested that the case against him emerged because he, a Sunni Muslim, applied for the post of chairman of the bar association - a post traditionally held by Christian Maronites.
"My father is not a member of any political party. He has defended all kinds of people, Lebanese people"
Ziad al-Maghrabi, the lawyer's son
"He is under arrest because he has become a formidable threat to the regime," says his son, Ziad al-Maghrabi.
"For 40 years, he has been filing lawsuits against prominent politicians in power, judges and lawyers. For three years, he has been leading a campaign for the integrity of the judiciary," he said.
Al-Maghrabi is known to have been taking on the defence of property owners of the once war-devastated Beirut downtown against Solidère, the firm entrusted to rebuild the area and the brainchild of powerful Prime Minister Rafiq Hariri.
Al-Maghrabi has also defended Christian fighters opposing the Syrian military presence and political influence in Lebanon.
"My father is not a member of any political party. He has defended all kinds of people, Lebanese people," said Ziad, the owner of an Internet provider.
Al-Maghrabi has become "a pain because he is constantly filing lawsuits, right, left and centre," said one lawyer. "He will not let go. He has made enemies everywhere."
Kamal Batal, director of a rights group, denounced al-Maghrabi's arrest as "he is not a threat to the community and is not attempting to flee."
The London-based rights group Amnesty International "considers the arrest of Dr al-Maghrabi to be arbitrary, and calls for his immediate release as a prisoner of conscience."
"Amnesty International fears that his arrest may be politically motivated and be meant to intimidate him as a human rights defender."
The New York-based Human Rights Watch has also called for the release of al-Maghrabi who has "steadfastly refused to remain silent about Syria's repressive role in Lebanon and about corruption within the judiciary."