By Suleiman Al-Khalidi

AMMAN (Reuters) - Thousands of protesters chanted the Arab Spring slogan "the people want the downfall of the regime" in Jordan's capital on Friday, as demonstrations against rising prices gather force in a country so far spared the brunt of Middle East unrest.

The mainly urban Muslim Brotherhood joined hitherto largely rural protests that have erupted in the last few days, raising the specter of lasting instability in the kingdom, a staunch U.S. ally with the longest border with Israel.

Friday's demonstration near the main Husseini Mosque in downtown Amman was peaceful, with unarmed police separating the demonstrators denouncing King Abdullah from a smaller crowd chanting in support of the monarch.

"Go down Abdullah, go down," the main crowd of about 4,000 protesters chanted as police, some in riot gear, largely stayed away from crowd.

Protests have turned violent in impoverished towns across the kingdom since Wednesday when the government imposed a hike in the price of fuel. Unemployed youths and demonstrators have attacked police stations, closed roads with burnt cars and torched government buildings.

One protester was killed on Thursday as a crowed tried to storm a police station in the northern city of Irbid. The provinces appeared to be quieter on Friday.

The Brotherhood's decision to back Friday's demonstration adds the voice of the country's best-organized opposition movement to the protests, although top Brotherhood figures did not appear in person.

"King Abdullah should take note of the situation by going back on the decision to raise prices. The Jordanian people are unable to shoulder more burdens," Brotherhood leader Sheikh Hamam Said said in a statement ahead of the protests.

Instability in Jordan would come at a dangerous time for the region, when Syria's war risks leaping borders and Israel is bombing Islamist-run Gaza.

OPPOSITION SEEKS REFORM

The slogan "the people want the downfall of the regime" has emerged as the main chant of Arab Spring demonstrations that toppled autocrats from Tunisia to Yemen, in many cases bringing to power elected Islamists allied to the Brotherhood.

In Jordan, an opposition of liberals and Islamists has generally sought reforms, rather than the overthrow of the 50-year-old king, in power since 1999.

A friend of the West, the monarch is seen by many Jordanians as a bulwark of stability, balancing the interests of tribes native to the east of the Jordan river with the increasingly assertive majority of Jordanians of Palestinian origin.

Abdullah accepted constitutional changes in August that devolved some of his powers to parliament and paved the way for a prime minister emerging from a parliamentary majority rather than one handpicked by him.

However, urban politicians say he has been too slow to adopt reforms, constrained by a tribal power base which sees change as a threat to political and economic benefits such as state jobs.

The Brotherhood is planning to boycott a parliamentary election set for January, arguing that rules were designed to safeguard tribal power by giving too many seats to rural areas.

Like many Arab states, Jordan has used government subsidies to appease the masses with cheap food and fuel, only to court unrest when the cash runs out.

Lifting the subsidies "deprives Jordanians of the minimum requirements of a decent living," Said said. "The King should speed reforms that restore power to the people to allow it put the corrupt on trial and restore embezzled money to the people."

(Additional reporting by Khaled Yacoub Oweis; Editing by Peter Graff)