GAZA CITY (Reuters) -- Islamic leaders in the Gaza Strip called on Friday for militants to kidnap Israelis and use them as bargaining chips to secure the freedom of thousands of Palestinians prisoners held in Israeli jails.

Human rights groups say up to 2,000 prisoners have joined an open-ended hunger strike to protest against jail conditions and thousands of Palestinians staged a rally in the Gaza Strip to support their cause.

"We should work hard to get (Israeli) prisoners in our hands in order to secure the freedom of our prisoners," Khaled al-Batsh, a senior member of the Islamic Jihad, told the crowd.

"I say to all armed factions, the way to free the prisoners is through swaps ... An arrest for an arrest, and freedom for freedom. This is the way," he said.

Israel last year freed some 1,000 Palestinians in return for the release of Gilad Shalit, a soldier seized in 2006 and held by the Islamist group Hamas in secret captivity for five years.

Human Rights groups say at least 4,700 Palestinians remain in Israeli jails, many of them convicted for violent crimes. Palestinian leaders say they should be treated as prisoners of war, something Israel rejects.

Ismail Haniyeh, the Hamas leader in Gaza, said Palestinian militant factions would "never abandon" the prisoners.

"The swap deal was a message to the (Israeli) occupation that the resistance and the Palestinian people will pursue every difficult avenue to break the chains of these heroes," he said.

"We are in a battle for the prisoners, and we will either win, or we will win," he added.

Friday's rally saw participants waving both the green and black flags of Hamas and the Islamic Jihad - a sign of growing ties between the two groups, which share the same Islamist ideology and advocate the destruction of the state of Israel.

Prisoners are seen as heroes in their communities and the mass hunger strike is putting pressure on the leadership to respond. Israel struck deals with two prisoners earlier this year to end their hunger strikes, but is resisting demands for further concessions.

At least two prisoners have been refusing food for more than eight weeks. A mass hunger strike by at least 1,200 was launched on April 17 and the Addameer prisoners' association has said a further 800 have since joined the movement.