Officials said on Monday that Israeli police and the military have started a search after the escape from Gilboa prison in northern Israel.
Five of the escapees belong to Islamic Jihad movement and one is a former commander of an armed group affiliated with the mainstream Fatah party, the Prisons Service said.
Arik Yaacov, the service’s northern commander, said the escapees appeared to have opened an hole from their cell toilet floor to access passages formed by the prison’s construction.
Haaretz newspaper said the six were cellmates and the tunnel they dug reached dozens of metres in depth.
The facility, about 4 km from the boundary with the occupied West Bank, is one of the highest-security jails in Israel and houses Palestinians convicted or suspected of anti-Israeli activities, including deadly attacks.
Four of the men were serving life sentences, a Palestinian prisoners organisation said.
Several Palestinian factions hailed the jailbreak.
“This great victory proves again that the will and determination of our brave soldiers inside the prisons of the enemy cannot be defeated,” said Fawzi Barhoum, a spokesman for Hamas.
A police spokesman said Israeli forces believed the fugitives might try to reach the occupied West Bank, where the Palestinian Authority exercises limited self-rule, or the Jordanian border some 14 km to the east.
One of the escapees was identified by the Prisons Service as Zakaria Zubeidi, a former commander of Fatah’s Al Aqsa Martyrs Brigades in the West Bank city of Jenin.