RAMALLAH, West Bank, Oct 27 (Reuters/Tom Perry) - For more than three years, the Palestinian Authority in the West Bank has tried to crush the Islamist movement Hamas.

Like Arab states which have tried for decades to do the same to the Muslim Brotherhood, Hamas's ideological parent, it has weakened but not wiped out the group.

And like the Arab governments that have tried to tackle such movements in the past, the Palestinian Authority risks repercussions. Hamas has warned of a backlash. Its armed wing has threatened retaliation for action against its fighters.

Mass arrests, arbitrary detention and torture, documented by Palestinian human rights groups, have been combined with other punitive measures in the Palestinian Authority's campaign against Hamas.

A student interviewed by Reuters said he was beaten unconscious in August during questioning over his involvement in a Hamas-linked university society. Under pressure from his family, he has decided to halt his activism. Like others, he would not be identified for fear of retribution.

They are always monitoring you, he said, referring to student spies whom he said report to the security services.

By its own admission, Hamas is a shadow of its former self in the West Bank. As the group deepens its control of the Gaza Strip, it says it barely exists as a political organisation in the West Bank. But, fortified by ideology, it is far from dead.

Hamas is a social movement, a social vibe, part of the fabric of society, said Fadel Hamdan, a Hamas leader.

The West Bank crackdown is a response to the mauling the Palestinian Authority suffered at the hands of Hamas in 2007, when it seized control of Gaza. President Mahmoud Abbas, who has the political and financial backing of Western states, wants to remove even a remote risk of that happening in the West Bank.

REVERSE THIS RAMPANT IMPUNITY

The PA security forces say Hamas continues to plot against Abbas's administration. A Hamas weapons cache was uncovered as recently as this month, spokesman Adnan Damiri said. He said any use of torture, banned under Palestinian law, was the result of illegal actions by individual interrogators rather than policy.

In Gaza, Abbas loyalists have their own narrative of brutality at the hands of Hamas. Fatah, the more secular party which dominates the Palestinian Authority (PA), is barely visible there. Many of its leaders have fled to the West Bank.

In the West Bank, Hamas is targeted at all levels, from the grassroots to the top. Being associated with Hamas could cost you your job. The group says the PA has fired hundreds of people, many of them teachers, for real or suspected Hamas ties.

Hamas sees an attempt to wipe it out by undermining the sources of popularity which contributed to its victory in 2006 legislative elections -- the last held by the Palestinians.

Any activity that puts you in touch with the people is banned, said Omar Abdel-Raziq, another Hamas leader.

Israel, the occupying power in the West Bank, still acts whenever it chooses against the group, which is deeply hostile to it. Israel views Hamas as a terrorist organisation, as do the Western states which back Abbas.

On Oct. 8, Israeli forces killed two members of the Hamas military wing who were suspected of killing four Jewish settlers. The Aug. 31 attack, on the eve of new peace talks with Israel, was as much a challenge to the Palestinian Authority, which opposes such violence, as to Israel.

Thousands of Hamas supporters turned out for the funerals -- a rare public show of strength. Hamas says dozens were later summoned for questioning by Palestinian security forces.

Palestinians associated with the group say being questioned or detained has become routine. Most are let go, but reports of torture during detention continue to filter out.

Human rights groups receive fewer complaints today than they did a year ago. But a report issued by 12 civil society groups in September said the security forces had resumed the practice.

Human Rights Watch, in an Oct. 20 statement, called on the Palestinian Authority to investigate two recent cases of alleged torture against men suspected of ties to Hamas.

President Abbas and Prime Minister Fayyad are well aware of the situation. They need to reverse this rampant impunity and make sure that those responsible are prosecuted, Joe Stork of Human Rights Watch said.

Reports of torture remain a source of concern for Western states whose support for the Palestinian Authority has included retraining its security forces, a diplomat said. The issue is raised regularly in bilateral meetings.

A WAITING GAME

The student interviewed by Reuters said he still suffers dizzy spells as a result of the beating in jail.

They hit me with fists to the head, face, he said.

He detailed more extreme cases of abuse suffered by his friends, 20 of whom had been detained. At least one, also interviewed by Reuters, had been subjected to stress positions.

The student said he was drawn to Hamas when he first started at university because he saw them as the people who helped the students most.

Many activists had opted to stand on the sidelines for now, he said. But Hamas differs from other parties: it has ideology and doctrine, so they can come back.

The Palestinian civil society organisations, in their September statement, said the PA had adopted a pattern of oppressive policies to stifle political dissent and to generate a sense of intimidation within Palestinian society.

Other measures, including the use of military courts to try civilians, have fuelled concerns the PA is sliding towards the style of government exercised by autocratic Arab states which have waged their own battles with Islamists.

Hamas says the campaign, while curbing its ability to function politically, has generated sympathy and strengthened a perception that the PA is collaborating with Israel.

The latest opinion poll from the Palestinian Center for Policy and Survey Research shows that Abbas would still beat a Hamas candidate were a presidential election held today. But there can be no elections in both the Gaza Strip and the West Bank until reconciliation between Hamas and Fatah.

Until then, contained by both the PA and Israel, Hamas is playing a waiting game.

This is a big organisation with wide support from the Muslim Brotherhood, Syria, Iran, Qatar, said Palestinian political commentator Hany al-Masri. Its strength will retreat, but it will not lose it completely.