GAZA, March 15 (Nidal al-Mughrabi/Reuters & JMCC) - Violence broke out after a rally for Palestinian unity in the Gaza Strip on Tuesday when Hamas supporters beat other participants with clubs, witnesses said, though the Islamist group denied being behind any violence.

Tens of thousands of Palestinians had answered a rallying call on Facebook to demonstrate in Gaza and the West Bank for reconciliation between Hamas Islamists and President Mahmoud Abbas's rival Fateh group.

About 2,000 demonstrators remained on the streets of Gaza as the rally wound down, defying calls to disperse by Hamas, which controls the territory. Hamas supporters then attacked the crowd, injuring at least a dozen people, witnesses said.

They attacked us, 400 to 500 Hamas men in plain clothes. I was beaten with a club, many others were beaten, said one activist, giving his first name, Ali. They dispersed the crowd.

Hamas denied the accusations, saying on its website that the crowd broke up after clashes between different youth groups.

Earlier in the day, stonethrowing clashes had broken out between Hamas supporters and the demonstrators, with several injuries reported.  

It was the first time Hamas's authority had been challenged in such a public manner since it seized control of Gaza from rivals loyal to the Western-backed Abbas in 2007.

Organizers of the rallies said revolts against authoritarian leaders elsewhere in the Arab world had inspired them to act.

Before the clashes Ismail Haniyeh, Hamas's leader in Gaza, said in a televised speech he respected the will of the people to establish a unified Palestinian leadership, and called on Abbas to meet him.

I invite President Abu Mazen (Abbas) and the Fatah movement to an immediate meeting, here in the Gaza Strip or in any other place, to launch a comprehensive dialogue, Haniyeh said.

This could be the doorway to achieving reconciliation.

BIG RALLIES IN GAZA

Rallies drew close to 100,000 protesters from various groups in Hamas-ruled Gaza, where unemployment is over 50 percent. The people want to end the division, one crowd chanted.

In the West Bank city of Ramallah, where only 4,000 joined a rally, Abbas renewed his call for presidential and parliamentary elections, which Hamas has repeatedly rejected.

Elections are the only way to end this ugly and unacceptable division, he told reporters after talks with the visiting president of Cyprus.

Salam Fayyad, Abbas's prime minister, told reporters in the West Bank that ending the split was a national priority.

I hope the call will be heeded ... we need to be able to bring this sad chapter to a close.

In Gaza, Hamas officials told other factions the rallies must end at sunset.

Organizers said Hamas security forces blocked all roads leading to the protest at sundown before club-wielding men in plain clothes, some riding motorcycles, attacked the crowd.

The spokesman of the Hamas-run interior ministry in Gaza, Ehab Al-Ghsain, said the report was a fabrication by Fatah officials with the aim of sabotaging internal security.

After its shock defeat of Fateh in the last Palestinian general election in 2006 and the collapse of a power-sharing bid, Hamas seized the Gaza Strip from Fatah in 2007.

Arab efforts to reconcile them made no progress.

Hamas does not support Fateh's policy of seeking peace with Israel to end the West Bank occupation and create a Palestinian state there and in Gaza. It refuses to renounce violence or to recognize the Jewish state.

The West regards Hamas as a terrorist organization and refuses to negotiate with it.

(Additional reporting by Ali Sawafta and Mohammed Assadi in Ramallah)(Writing by Jeffrey Heller and Nidal al-Mughrabi, editing by Tim Pearce)