CAIRO (Reuters) - While security forces round up the Muslim Brotherhood, Egypt's second largest religious party has warned the State against trying to wipe out political Islam entirely during its toughest crackdown in decades.

The Nour Party, a Salafist group that backed the military's removal of President Mohamed Mursi last month, is now also feeling the heat, its leader Younes Makhyoun told Reuters.

Members of his pacifist party - which follows an austere interpretation of Islam - have been beaten, harassed and turned over to the police in recent days, simply because they wear beards as a sign of their religious observance, he said.

With at least 900 people killed in a week, Makhyoun cautioned against an arbitrary campaign targeting Islamists, saying this would drive some underground. "This will be a dangerous path and make many disavow the tools of democracy, and perhaps resort to other methods," he said in an interview.

Political Islam could not be "uprooted", he added. "If anyone is thinking about excluding it, that is of the utmost stupidity."

The Brotherhood's main rival, the Nour Party turned strongly against the much older Islamist group earlier this year, joining liberals who accused Mursi of staging a power grab.

When Mursi was overthrown on July 3, Nour endorsed an army-backed transition plan, lending Islamist support to the new order as the government promised an inclusive process. But the party began to distance itself from the government after dozens of Brotherhood supporters were gunned down by security forces.

Makhyoun, a dentist who took over the party leadership last year, said the government had to assure Egyptians that freedoms won by the uprising that toppled Hosni Mubarak in 2011 would not be rolled back following the bloodiest week in Egypt's modern history.

"We need guarantees from the authorities to the Egyptian people: that the gains of the Jan. 25 revolution cannot be violated, especially in the field of freedoms, human rights and freedom of expression," he said.

Head of a party born out of the uprising, Makhyoun described threats to freedom including a revival of the Mubarak-era political security apparatus as the state fights the Brotherhood, which ruled for a year until Mursi's fall.

Some Nour members, angered by events and agitated by Brotherhood rhetoric, had broken party ranks, he said, adding to indications that Egypt's established Islamist groups may be losing their grip on supporters.

Since the crackdown began, more than 1,000 Brotherhood activists including its leaders have been arrested while 100 members of the security forces have also died in the last week.

Makhyoun warned against violence and counter-violence at a time when arms have been freely available in neighbouring Libya since the fall of Muammar Gaddafi two years ago.

"Enormous quantities of weapons" had entered Egypt from Libya since 2011, he said. A compromise solution was now more complicated than ever but the only way out of the crisis, he added. "If Egypt sinks, it will sink with everyone aboard."

STATE SECURITY

The Nour Party, the most successful political newcomer after Mubarak's downfall, tried to take the middle ground in the last months of Mursi's rule. It tabled proposals for defusing political tensions and boycotted pro-Mursi rallies.

That reduced Mursi's allies to smaller Islamist parties including the Gamaa Islamiya, a once armed Salafist movement.

After Mursi's downfall, Nour exercised major influence in shaping the interim government, vetoing two candidates for prime minister. Analysts say the party has paid a price as its approach eroded its standing among some Islamists.

Makhyoun said the party had faced "enormous pressure" from its youth base to take part in the Brotherhood-led protests. It had been difficult to keep them in check as Mursi's allies used religious rhetoric to rally young Islamists.

"There was great difficulty in convincing them in this charged, emotional atmosphere," Makhyoun said. "There are some people who broke away - this is something natural," he said. "I am confident that when matters become clear, they will return."

Makhyoun traced the crisis to the violent language he said the Brotherhood and its allies had used on the eve of Mursi's downfall. Leaders of Gamaa Islamiya, which waged an insurrection in the 1990s against the State before joining the political process, had said Mursi's opponents should be crushed.

"We were accused of treachery: everyone who was against them was a traitor. This is difficult language and could take the youth to the path of violence, no doubt," he said.

Brotherhood activists involved in violence should be put on trial, he said. But "random arrests of all Brotherhood members is a mistake", he said.

Since Mursi's downfall, the Interior Minister has announced the revival of a political security apparatus, stirring fears that agencies used in Mubarak's days to suppress the opposition would be used against the Islamists.

Makhyoun listed that as one of his main concerns, together with the reimposition of the state of emergency, the closure of Islamist TV stations the day Mursi was toppled, and the mobilisation of "thugs" by the state.

"There are checkpoints manned by thugs who specifically harass those with beards," Makhyoun said. "This has happened a lot to members of the Nour Party."

"People are asking us every day: 'What do you think? Has State Security returned again?" said Makhyoun. "We don't know their intentions - only God knows them - but what we can say is there are steps and signs that cause concern."

(Editing by Michael Georgy and David Stamp)