Police for Hamas, which rules the Gaza Strip, have not yet commented on the explosions in different areas of Gaza City. There was no immediate confirmation of any casualties.
A security source said three of the cars belonged to members of Hamas and two to Islamic Jihad members.
There have been growing signs of internal unrest in the Gaza Strip, with Hamas engaged in a power struggle with smaller extremist groups.
A spate of bomb attacks has targeted public buildings and officials as well as international organisations.
Rocket fire from Gaza into Israel has also been claimed by Salafist extremists claiming links to the Islamic State group.
Salafist groups have been angered by a Hamas crackdown and claimed in recent weeks that around 100 of their members or supporters were behind bars.
They also criticise Hamas for what they see as its lack of zeal in enforcing Islamic law as well as for its truce with Israel since last year's war in the Gaza Strip.