By Suleiman Al-Khalidi

AMMAN (Reuters) - Jordan's main Islamist opposition is preparing to flex its muscles in a march on Friday that could be its biggest demonstration since Arab Spring-inspired protests last year against the slow pace of political reform.

The "Friday to Rescue the Nation" rally called by the Muslim Brotherhood, the largest opposition party, in the capital Amman will press for broader political representation and a more democratic parliament.

But plans by pro-government groups to stage a counter-demonstration have raised fears of clashes.

"We are talking about a new phase after 20 months of continued popular protests and unwillingness to listen to our demands for reform," Zaki Bani Rusheid, the deputy head of the Muslim Brotherhood, told Reuters.

Jordan has had nearly two years of peaceful street protests by Islamists, tribal figures and leftists, inspired by the Arab Spring uprisings, but they have focused on reforming government and limiting King Abdullah's powers rather than ousting him.

Jordan's Islamists are highly organized and have been mobilizing supporters by distributing leaflets at mosques. Their main rallies usually attract tens of thousands of people.

"We are calling for real reforms that restore power to Jordanian people and curb the powers of those who have seized power and influence for decades," Bani Rusheid said.

He accused the authorities of creating a climate of "incitement and fear" to prevent people from other areas turning up to march, after a backlash in most pro-government media.

Some tribal and leftist groups have said they will join the march. Pro-government groups, widely suspected of being directed by the authorities, have said they will rally in the same area.

Loyalists tied to the country's powerful intelligence apparatus have been blamed for disrupting past Islamist-led gatherings and there are fears of clashes on Friday.

Bani Rusheid said any attempt by the security forces to provoke a confrontation would backfire.

"The inclination to artificially instigate a confrontation is a gamble with dangerous consequences and no one would be able to predict its outcome. They have tried in the past to provoke us and get us to act irresponsibly but all attempts have failed," he said.

'ROYAL INITIATIVE'

Jordan has tolerated protests in provincial towns inhabited by native Jordanians from East Bank tribes, who are used to preferential access to state jobs. Those protests have been driven largely by concern over dwindling benefits.

But the mostly urban Islamists, with a strong following among Jordanians of Palestinian origin, are viewed by the political establishment as a threat.

Bani Rusheid has a wide following among poor Jordanians and has been the focus of vitriol by pro-government hardliners.

The Muslim Brotherhood refuses to take part in an election for parliament to be contested within months under an electoral law passed in July by a rubber stamp parliament.

"When the security forces failed to make us participate in elections on their terms they wanted to take revenge on the Islamic movement with the same old methods that existed before the Arab Spring. They forget the world has changed," he said.

The Islamists say the law gives pro-government tribes in sparsely populated provincial areas a much bigger allocation of parliamentary seats than their own strongholds in cities where Palestinians dominate.

Jordan's political elite has watched with trepidation as Islamist parties in Tunisia, Egypt and Libya have gained ground, seeing their rise as a sign of the future empowerment of Jordanians of Palestinian origin, a majority of the population, who are unrepresented politically but are the backbone of the economy.

Bani Rusheid said only King Abdullah could stand up to the old guard bent on preserving their privileges. The king says his reformist agenda has been frustrated by conservative politicians who hold extensive power within the security establishment.

"We are awaiting a royal initiative. King Abdullah is the only person able to rein in these security forces and get the country out of this crisis and to achieve a major political breakthrough from a grinding deadlock," he said.

(Editing by Janet Lawrence)