Police held back crowds marching from parliament to Ala-Too Square, chanting "Bakiyev resign" and waving red opposition flags in protest.
Police said up to 6,000 people had gathered in the central square, although a Reuters reporter at the scene estimated that 15,000 were present.
A reporter from the Associated Press said that as many as 30,000 people attended the demonstration at its peak.
Omurbek Tekebaev, one of the man opposition leaders, told demonstrators: "Bakiyev was elected to carry out reforms. He has done nothing in a year and a half."
The opposition has been urging Bakiyev, who came to power in April 2005, to fulfil his election promise to conduct constitutional reforms and curtail presidential powers. They want parliament and the cabinet to be given greater authority.
Eshimkhanov said Bakiyev had said in talks on Tuesday that he would dissolve the cabinet immediately and allow parliament to form a new one. The president also promised to submit constitutional reforms to the parliament on Thursday, an opposition demand.
Regional confrontation
In an apparent attempt to head off civil unrest, Bakiyev told parliament he would bring forward the presentation of the new constitution to next Monday.
"We either adopt the constitution through the parliament, or in December people will decide what form of governance they need," Bakiyev said.
The Kyrgyz capital saw large-scale riots following the March 2005 overthrow of Askar Akayev, the former president who fled to Russia.
Bakiyev was elected months later, but his presidency has been marred by high-profile killings, prison riots and battles for control of lucrative businesses.
Kyrgyzstan, hailed in the 1990s as an "island of democracy" in Central Asia, has seen deteriorating public security, economic problems and political tension.
The impoverished former Soviet republic has teetered on the brink of political crisis since Bakiyev came to power in 2005 after riots ousted his long-serving predecessor, Askar Akayev.