Many enjoying a day by the sea were injured by the Gaza City blast
A powerful explosion inside a car travelling past a beach in the Gaza Strip has killed four Hamas militants and a young girl, medics have said.
At least 20 other people, including many enjoying a day by the sea, were said to be injured by the explosion on Gaza City's coast road.
Leaders of the Islamist movement have blamed Israeli collaborators in Gaza.
It was the third bomb attack in a day - one of the bloodiest since Hamas and Israel agreed a ceasefire last month.
Earlier on Friday, a bombing outside a cafe in Gaza City killed one person - reportedly the perpetrator - and injured at least three others, Hamas officials said.
It was unclear why the area was bombed, but there have been attacks on internet cafes, music shops and Christian institutions in Gaza in recent months.
Officials also reported a bombing near the home of Marwan Abu Ras, a Hamas politician and academic. No-one was injured.
The explosion on Friday night at a major junction besides Gaza City's beach killed a commander of Hamas' military wing, the Izzedine al-Qassam Brigades, and three other members, hospital officials said.
We have information that some elements are planning to carry out bombings against the interests and leaders of Hamas in order to sow anarchy
Khalil al-Hayya Hamas leader
The five-year-old girl killed in the blast had been on her way to the beach with her family, they added.
Afterwards, Hamas did not explicitly accuse its bitter rival, Fatah, of carrying out the attack, but it did imply it, the BBC's Aleem Maqbool says.
Khalil al-Hayya, a Hamas leader whose son was wounded in the blast, blamed Palestinian groups "who collaborate with the enemy [Israel]".
"We have information that some elements are planning to carry out bombings against the interests and leaders of Hamas in order to sow anarchy," he told the Reuters news agency.
Aftermath of the bombing
The Izzedine al-Qassam Brigades, however, were more explicit and blamed "members of the fugitive party" - a derogatory term for Fatah.
Hamas seized control of the Gaza Strip in June 2007 from the Fatah movement of Palestinian Authority President Mahmoud Abbas.
Since then, Hamas's forces have arrested many Fatah supporters.
Most have been afraid to challenge the Islamist movement and so while there has been dissent, factional killings have decreased considerably, our correspondent says.
In response to this latest internal violence, Hamas immediately set up checkpoints throughout Gaza City.
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