The Muslim Brotherhood's political wing, the Islamic Action Front, called for broader political representation and a more democratic parliament.
People at the protest chanted: "The people want to reform the regime."
On Thursday evening, the king dissolved parliament and called early elections, though he did not specify a date.
He has said he wants polls to be held by the end of the year.
The IAF said it expected 50,000 people to take part in the protest outside the capital's al-Husseini mosque after Friday prayers. But the BBC's Wyre Davies put the turnout at 10,000 people, which he said was the biggest protest seen in Jordan for several years.
Video footage showed protesters chanting slogans and waving flags. The AFP news agency quoted people as shouting: "We demand constitutional reform before the people revolt. The people want to reform the regime."
A counter-rally, in support of King Abdullah, which organisers had predicted would attract 200,000 supporters, was cancelled late on Thursday in order to prevent clashes between the two groups.
Earlier, police said they had detained eight people travelling towards the rally and had seized three vehicles containing sticks, knives and guns.
Friday's protest in the capital and the dissolution of parliament come amid mounting opposition anger at the electoral law passed in July by the government of the conservative Prime Minister Fayez al-Tarawneh.
The electoral law increased the number of seats in the House of Representatives from 120 to 150 seats and gave the electorate two votes - one for a district representative and one for national level lists that include political parties - replacing the single non-transferable vote.
The IAF's leader, Hamza Mansour, dismissed the legislation as "just a cosmetic change meant to buy time and insufficient for real reforms".
Opposition parties demanded that 50% of seats be allocated to party lists, but the new electoral law gave them just 27 seats, or 18%.
They also complained that the new law would strengthen supporters of the king by allocating three more seats for women from Bedouin districts.
This, they argued, would continue to marginalise Jordanians of Palestinian origin - who make up 60% of the population but have little political power - in favour of those descended from Jordan's original Bedouin inhabitants - whose tribes dominate the government and security forces and are the bedrock of the Hashemite monarchy.
Traditionally, many of the IAF's supporters have been Jordanians of Palestinian origin.
The opposition also demanded that parliament, rather than the king, should have the right to appoint and dismiss the prime minister.
Despite the call for reforms, Jordan has so far avoided the unrest and political upheaval that rocked much of the Arab world last year. Protests have been relatively small and have not gained the same level of political momentum as those in Tunisia, Egypt, Libya, Yemen and Syria.
Correction 25 January 2013: A previous version of this story incorrectly stated that members of the Jordanian security forces were allowed to vote for the first time.
Thousands of Jordanians have attended a protest demanding political reforms in Amman, hours after King Abdullah called early parliamentary elections.