Hundreds of people have begun to gather in the Jordanian capital of Amman for a pro-reform rally, despite King Abdullah's decision to dissolve parliament one day earlier, a move the monarch hoped would head off a large protest Al Jazeera reproted.
At least 2,000 police had been deployed for the demonstration which was due to follow the main weekly Muslim prayers.
The rally is being organised by the Islamic Action Front, the Jordanian wing of the Muslim Brotherhood and the country's main opposition group. The group is predicting a turnout of as many 50,000.
King Abdullah had hoped that dissolving the parliament and calling for early elections would weaken enthusiasm for the protest. The king did not announce a date for early elections, but said he hoped to hold polls by the end of 2012.
A demonstration in support of the king was called off over fears of unrest as it would have coincided with the opposition rally, organisers said.
"We have postponed indefinitely our demonstration scheduled at the same time as the Muslim Brotherhood's to avoid any problems," said Jihad al-Sheik, head of an internet-based youth group that organised the event.
The cancellation came "after a request to that effect from the director of general security, Hussein al-Majali, MPs and tribal leaders" to prevent unrest.
'A tremendous miscalculation'
The Brotherhood and a coalition of tribal and other groups have been pressing the monarch to speed up what they consider to be the slow pace of political reform.
They are also angry with an electoral law passed last July, which preserves a system that marginalises the representation of Jordanians of Palestinian origin, on whom Islamists rely for their support, in favour of native Jordanians, who tend to support the king.
The opposition says it will boycott upcoming elections, as it did with the 2010 poll, unless its demands for wider representation are met.
Numerous other demonstrations have taken place in Jordan since January 2011 to call for political and economic reforms and demand an end to corruption.
In an exclusive interview with AFP news agency last month, the king said a decision by the Islamists to boycott the vote was "a tremendous miscalculation."
"As constitutional monarch, my mandate is to be the umbrella for all political groupings and all segments of our society, and as part of that responsibility, I am telling the Muslim Brotherhood that they are making a tremendous miscalculation," he said.
"The countdown to the elections has already started. Registration is under way. We have already crossed the one-million person mark. Parliament will be dissolved. The elections date will be announced. And we will have a new parliament by the new year."
King Abdullah has ordered parliament to increase seats reserved for party candidates, urging the Islamists to take part in the polls. MPs raised the number from 17 to 27, but failed to satisfy opposition groups.