Thousands of Islamists demonstrated in central Amman to call for reforms on Friday, just hours after King Abdullah II dissolved parliament and called early polls without any major political change.
"We demand constitutional reform before the people revolt. The people want to reform the regime," they chanted in the demonstration held outside Al-Husseini mosque in downtown Amman.
AFP correspondents estimated the crowd to number 15,000.
"Democratic electoral law, constitutional changes, parliamentary governments, independent judiciary, constitutional court, effective anti-corruption efforts and preventing security services from interfering in political life," read a large banner carried by protesters spelling out their demands.
Police said they prevented a group of youths from attacking the demonstrators, after they arrested eight people who were found to be carrying in three minibuses heading into central Amman.
"We have been protesting for more than 20 months and you still do not understand our demands. We do not like demonstrations but we love Jordan," read another banner carried by the protesters.
The demonstration was the latest in a series of protests in Jordan since January 2011 to call for political and economic reforms and to demand an end to corruption.
The king decided to dissolve the chamber of deputies on Thursday and to call early elections, the royal palace said. It gave no date, although the monarch has said he wants polls to be held by the end of 2012.
Signup to our newsletter and follow us on Facebook and Twitter!
But the opposition Muslim Brotherhood said it was going ahead with its planned rally in central Amman after Friday prayers, with the group predicting an estimated 50,000 supporters would attend.
A rival demonstration in support of the king's plans, due to be held in the same location as the opposition protest, was "postponed indefinitely ... to avoid any problems," said an organiser, Jihad al-Sheik.
Around 200,000 people had been expected to turn up to show their support for the king's efforts to bring in reforms.
The Brotherhood has said it would boycott polling as it did Jordan's last elections in 2010 to protest the lack of meaningful reforms. It demands a parliamentary system where the premier is elected rather than named by the king.
"I am telling the Muslim Brotherhood that they are making a tremendous miscalculation" with their threat to boycott, the king told AFP in an exclusive interview last month.
Other opposition parties have also said they were considering a boycott over a new electoral law under which voters cast two ballots -- one for individual candidates in their constituencies and one for nationwide party lists.
The king ordered parliament to increase the number of seats reserved for party candidates in a bid to persuade the Islamists to take part. MPs raised the number from 17 to 27, but that did not go far enough for the opposition.
"This elections law is not perfect. We all understand that. But there is no better consensus on an alternative. What is critical is that we keep going forward," the king said.
"So I am telling the Muslim Brotherhood, you have a choice. To stay in the street or to help build the new democratic Jordan."
The Islamists boycotted Jordan's last elections in protest at constituency boundaries, saying they over-represented loyalist rural areas at the expense of urban areas seen as Islamist strongholds.