Youngsters carrying the Lebanese flag late on Sunday converged on Martyrs' Square, where the opposition has called for a peaceful sit-in on Monday in defiance of the ban, which comes into force at 0300 GMT.
"We are going to hand out blankets, we are staying here," one of the demonstrators said by loudspeaker.
Hundreds of heavily armed troops deployed with jeeps and trucks at all the main crossroads leading to the square.
And hundreds of protesters stopped from getting to the square blocked nearby crossings, some of them shouting: "We don't want any other army than the Lebanese army!"
Interior Minister Sulayman Franjiyah earlier on Sunday outlawed all public demonstrations.
The minister said the ban was "due to the current circumstances, in the supreme national interest and with a view to the requirements of protecting civil peace".
"All security forces are asked to take all necessary measures to protect security and order, and to ban demonstrations and gatherings on Monday," he said.
Opposition sit-ins have been held nightly for the 12 days since the assassination of five-time prime minister Rafiq al-Hariri.
"I hope all the MPs will stand by their duty, defending the interests of the people in confronting this tyrannical fascist regime"
Lebanese opposition leader
Leaders of the opposition maintained their call for Monday's
sit-in after the government announced the ban.
Druze leader Walid Jumblatt promised there would be defiance on Monday.
"We are going ahead. They cannot prevent us from going down peacefully, democratically and paying tributes to Rafiq al-Hariri on the day of the national parliamentary debate where our main aim is to ask who killed al-Hariri," he said.
"I hope all the MPs will stand by their duty, defending the
interests of the people in confronting this tyrannical fascist
regime, because it's no more than two weeks since al-Hariri was killed," Jumblatt added.
Lebanon has launched an investigation into the assassination
but rejected calls for a full international investigation.
Figures from across Lebanon's disparate opposition movement
have seized on public fury at al-Hariri's killing to demand that
Syria pull out its troops and intelligence services and that the
Beirut government it backs resign.
Syria plays a powerbroking role in Lebanon, where it keeps
Syria has about 14,000 troops
deployed in Lebanon
Opposition deputies and many ordinary Lebanese have held
Syria and the Lebanese authorities either directly or indirectly responsible for al-Hariri's death along with 17 other people in a car bombing.
Damascus denies any role in al-Hariri's killing and has described the assassination as terrorism.
Government and Syrian loyalists, meanwhile, planned to
descend on central Beirut to protest against US Deputy
Secretary of State David Satterfield's visit to Lebanon as part of growing international pressure for Syria to withdraw its troops.
There were fears of clashes between the two groups.