Hamas is arresting suspected jihadist activists in Gaza, which the militant Islamist movement controls, after what a security source said were suicide bomb attacks on two police posts.
Three police officers were killed in the attacks, which are thought to have been carried out by extremists with links to the Islamic State (IS) group.
Gaza's interior ministry declared a state of emergency after the bombings.
Hamas leader Ismail Haniya vowed to hold those responsible to account.
The bombings would "not be able to undermine the stability and steadfastness of our people", he added.
According to reports from Gaza, two police officers were killed in Tuesday night's first attack, in which a motorcycle exploded as it approached a checkpoint.
A second, similar, attack less than an hour later killed another police officer.
Israeli military sources immediately denied that they had launched any operations in Gaza and suspicion fell on Salafist jihadist factions, who have periodically challenged Hamas' authority in the Palestinian territory.
Hamas recently released several prisoners from detention, as a gesture of goodwill, including some members of militant groups affiliated with IS.
The BBC understands that at least one of the suspected bombers had been previously detained and was one of those recently released.
The BBC's Wyre Davies in Jerusalem says these rare attacks are embarrassing for Hamas and could be interpreted as a renewed effort by Salafist jihadists to confront the group.