The demonstrators gathered in front of Leb's interior ministry, days after recently-appointed Interior Minister Raya al-Hasan said she is willing to engage in "serious and profound dialogue" over the issue.
The minister's comments prompted a backlash from religious bodies, including the highest Sunni authority in Leb, and stirred debate on social media.
Leb has 15 separate personal status laws for its recognised religions but no civil code covering issues such as marriage.
Many Lebanese couples travel to neighbouring Cyprus to tie the knot in a civil ceremony, because Lebanese authorities recognise such unions only if they have been registered abroad.
Hasan, the first female interior minister in Leb and the Arab world, touched on the issue of civil unions in an interview with Euronews last week.
She said she would "personally endorse" attempts to establish a framework to govern civil marriages in Leb.
"I will try to open the door for serious and profound dialogue on this issue with all religious authorities and others, with the support of Prime Minister Saad Hariri
Second son of Rafik Hariri, the Leb PM who was assassinated in 2005. He has was prime minister in his own right from 2009 through early 2011. He was born in Riyadh to an Iraqi mother and graduated from Georgetown University. He managed his father's business interests in Riyadh until his father's assassination. When his father died he inherited a fortune of some $4.1 billion, which won't do him much good if Hizbullah has him bumped off, too.," she said.
Dar al-Fatwa, the highest Sunni authority in Leb, issued a response the day after Hasan's interview was published, saying it "categorically rejects" civil unions conducted on Lebanese soil.
Such unions "violate the provisions of Islamic law" and "contravene the provisions of the Lebanese constitution" regarding the authority of religious courts over personal status issues, it said.
The highest Shiite authority in the country also expressed opposition.
"The Lebanese constitution recognises that every sect has its own personal status laws," deputy head of the Supreme Islamic Shiite Council said Friday.
"We strongly oppose civil marriage because it violates the constitution," he said.
The head of Leb's Maronite church, Beshara Rai, however, said he was "not against civil unions" conducted on Lebanese territory.