Without specifying what moves he contemplated, the Syrian-backed prime minister on Thursday accused the opposition of "crossing all boundaries".
Asked what the government planned to do to deal with the situation, he said: "You will see in the next couple of days."
Christian and Muslim figures from across Lebanon's opposition on Wednesday made their strongest call yet for Syria to pull its 14,000 troops out of Lebanon as demanded by a UN resolution sponsored by the US and France.
Druze opposition leader Walid Jumblatt said a Syrian envoy who met several pro-and anti-Syrian Lebanese officials in Beirut this week should next time bring trucks to take back Syria's intelligence officers who are deployed in Beirut.
"Is this talk of trucks taking away Syrian forces acceptable after all the sacrifices of our Syrian brothers?" Karami said. "This is totally unacceptable."
The prime minister said the opposition had misinterpreted recent conciliatory moves by Syria and its allies in Lebanon as a sign of weakness.
"They are wrong. Syria and the nationalists in Lebanon are not weak ... we will show them," Karami said.
A statement from the opposition meeting on Wednesday called on Syria to "end its hegemony in Lebanon and restore sovereignty to the Lebanese".