According to AFP citing medical sources, a BMW detonated when Hamdan opened a door to the auto.
An official in the Palestinian Islamist movement Hamas was wounded in a vehicle bomb blast in the southern city of Sidon on Sunday, military and medical sources told French news agency AFP.
Mohamad Hamdan, also known as Abou Hamza, was identified as the target of the bombing.
A medical source at the scene said Hamdan, also known as Abu Hamza, suffered serious wounds to his legs while opening the door to his vehicle, and was transported to hospital.
The blast took place around noon (1000 GMT), the Lebanese army said in a statement.
The statement added that the blast took place as he opened his auto in Al-Bustan Al-Kabir neighborhood in Sidon.
Local media reported that Hamas had released a statement denying that any of its officials had been killed in the attack.
These units - which include Hamas and rival Palestinian faction Fatah - have fought several battles with other groups inside Ain al-Hilweh.