Around 10 people were wounded in the blast that blew up Mr Tueni's armoured 4WD car as it travelled in the Mekalis area.
Witnesses said at least three people inside the car were killed.
Mr Tueni was elected to parliament in this year's election, and is a firebrand critic of Syria.
Several cars were set ablaze and nearby shops and buildings suffered some damage.
Police and soldiers cordoned off the area as rescue workers ferried casualties to hospitals.
Lebanon has been rocked by a series of bombings and assassinations since the February 14 killing of former prime minister Rafiq Hariri.
The attack comes just hours before a United Nations team investigating Mr Hariri's death is to present its findings to the UN Security Council, and is expected to outline Syria's alleged role in the murder.