There was a strong security presence, but police stayed on the sidelines and even gave bottles of water and juice to the protesters.
"We are demonstrating today against the official bullying and to demand reforms," leading trade unionist Maisara Malas told AFP.
"We seek regime reforms. We want a true parliamentary monarchy. The monarchy should not dominate parliament."
Hamzah Mansur, chief of the Islamic Action Front, the political arm of the Muslim Brotherhood in Jordan, told the crowds "reforms have become a necessity that cannot be delayed."
"We want immediate constitutional change to help create productive governments and a truly representative parliament. These are the demands of all Jordanians."
Holding national flags and pictures of King Abdullah II, the protesters chanted: "The people want to reform the regime, dissolve parliament and amend the constitution."
They carried banners reading "we have no place for corruption and the corrupt," and "enough bullying, we want democracy, not a security mentality."
They also called for scrapping amendments to the 1952 constitution, which was promulgated by King Abdullah II's grandfather King Talal.
The document has been amended 29 times, giving greater power to the monarch and weakening the legislature, experts say.
Police said 6,000 people took part in the demonstration, the largest since last month, while organisers put the figure at more than 10,000 supporters of the Islamist movement and 19 political parties.
A top security official told AFP that more than "3,000 members of different security services are currently in downtown."
Police gave water and juice bottles to the protesters, who also demonstrated to "denounce violence," in which eight people were hurt last week when a baton-wielding mob waded into a pro-reform march in Amman.
"Three people have been arrested over their suspected involvement in the violence last Friday," another senior security official told AFP.
The government has condemned the violence and opened a probe into the incident.
Meanwhile, more than 100 supporters of the Hashemite royal family gathered outside Al-Husseini Mosque, in the heart of the capital.