The radical United Front opposition, led by former Premier Felix Kulov, said it expected to gather 50,000 supporters for open-ended protests on Ala-Too square near the government building.
Columns of protesters are being formed at seven sites in Bishkek to move toward the square, where the opposition set up tents and traditional Kyrgyz huts Tuesday night.
President Kurmanbek Bakiyev, who came to power in an alliance with Kulov as a result of a violent uprising in 2005, signed fresh constitutional amendments Tuesday consenting to curtail his powers in an apparently futile attempt to prevent a further escalation of the situation.
The Kulov-led opposition accuses the president of failing to improve living standards, curb corruption and introduce democracy to the Central Asian republic. Kulov also argues that part of the votes that brought Bakiyev to presidency belonged to him, which, he said, put in doubt the legitimacy of the president's office.
The United Front has been joined by the more liberal opposition movement For Reforms, which unites 20 parties and non-governmental organizations.
Kyrgyzstan's Deputy Interior Minister Temirkan Subanov said that about 4,000 law enforcement officers had been summoned to provide security during the protests.
Law enforcement officers are dressed in police uniform to prevent confusion after the opposition distributed camouflage uniforms to its supporters "to imitate security officers' defection to the opposition or to organize provocations under the guise of the uniform," Subanov said.
