Main Lebanese opposition figure Walid Jumblatt said the protests would go ahead.
"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," Jumblatt said.
"Tomorrow residents of Beirut and those coming to it from across Lebanon will hold a sit-in in Martyrs Square," the opposition said in a statement, issued shortly after the government announced the ban on Sunday.
Opposition figure Ilias Atta Allah said: "The ban does not concern us, we are only holding a peaceful sit-in which will be maintained.
"Let them arrest us."
Interior Minister Sulayman Franjiyah had earlier outlawed all public demonstrations, on the eve of rival rallies called by the opposition and pro-government parties sympathetic to Syria.
"All security forces are asked to take all necessary measures to protect security and order, and to ban demonstrations and gatherings on Monday," Franjiyah said.
He said the ban came "due to the current circumstances, in the supreme national interest and with a view to the requirements of protecting civil peace".