The ban on smoking in places like restaurants and pubs went into effect Monday.
The restaurant owners say the ban will harm their businesses, where many Lebanese are used to smoking cigarettes and puffing on water pipes. Dozens demonstrated in a restaurant district north of Beirut.
Lebanon's parliament passed the law banning smoking in closed public places last year.
Violators will be subject to fines of about $90 to the client and $900 to the offending owner.
The law also bans cigarette advertising.
Smoking is common in Lebanon. A recent study showed about half of Lebanon's adults are smokers.
Officials say Lebanon spends about $350 million annually for treatment of smoking-related diseases.