Thousands of protesters made unprecedented calls on Friday for Jordan's King Abdullah II to go, as police blocked them from heading to the royal palace to vent their anger over big fuel price increases.

"Freedom, freedom, down with Abdullah," chanted crowds that AFP estimated at around 10,000 people, including Islamists, leftists and youth activists.

Publicly insulting the king or calling for his overthrow is punishable by imprisonment in Jordan so the demonstrators' slogans were a major departure for a kingdom that had previously been spared protests on the scale of other countries swept up in the Arab Spring.

"The people want the fall of the regime," the protesters shouted angrily outside the Husseini Mosque in the heart of the capital, using the ralling call of the uprisings that swept aside veteran rulers in Tunisia, Egypt, Libya and Yemen last year.

"Abdullah, reform or leave, you have lost legitimacy," they chanted. "God is greater than injustice... Abdullah your era is gone."

Organisers said more than 25,000 people took part in the demonstration. Police put the number at 3,000.

In the face of the scale of the protests, the king cancelled a visit to London he was due to make next week, the British foreign ministry said.

US Secretary of State Hillary Clinton telephoned the king to voice Washington's strong support, the royal palace said.

"Clinton praised the king's road map for political reform as well as government economic reform efforts. She also stressed the importance of Jordanian-US partnership," a palace statement said.

Police had prevented the demonstrators from heading to the palace which lies around eight kilometres (five miles) from the Husseini Mosque but there were no clashes, an AFP correspondent reported.

Protesters held up banners saying: "Playing with prices means playing with fire," "This is a real revolt against corruption" and "No reform without political and economic change. Long live the revolt of Jordanians."

The deputy head of the main opposition Muslim Brotherhood, Zaki Bani Rsheid, told AFP: "The numbers calling for the fall of the regime are growing because of wrong polices that reject people's demands.

"This cannot and should not be ignored. The regime must reform before it is too late."

The protesters had said they planned to hold another demonstration at around 7:00 pm (1600 GMT) near the interior ministry, but only around 100 showed up.

Some 2,000 anti-riot policemen sealed off the area, as some 200 loyalists of the monarchy clashed with the small knot of anti-government protesters forcing them to flee.

Demonstrations were also held just outside Amman in the Baqaa Palestinian refugee camp and in the cities of Tafileh, Karak and Maan south of the capital, and Irbid and Jerash to its north.

The wave of protests erupted on Tuesday in response to the announcement of a 53 percent increase in the price of household gas and a 12 percent rise in the price of petrol.

But as with other Arab Spring protest movements the economic grievances have swiftly taken on political overtones.

The Muslim Brotherhood has demanded that the king cancel the price hikes and postpone a snap general election called for January 23, which the group has said it will boycott.

The Islamists say that reforms promised by the king do not go far enough in establishing a constitutional monarchy with a prime minister elected by parliament rather than appointed by the king.

Washington, which has said it is monitoring developments closely, called on Americans to avoid areas where demonstrations are being held.

Saudi Arabia too urged its citizens to stay away from public gatherings, the official Saudi Press Agency reported.