Some 2,000 members of Public Parliament, a broad coalition of opposition parties and NGOs, issued a resolution after a meeting in the capital, Bishkek, setting the deadline for the authorities until March 2009.
The opposition "proposes the president take steps to create a new political system by March 2009," the resolution said. "Otherwise, we will seek President [Kurmanbek] Bakiyev's resignation by means of mass protests."
More radical opposition groups in the ex-Soviet republic earlier said they planned rallies in the capital in December.
Kyrgyz authorities have rejected opposition demands saying the constitution can only be amended via a referendum.
Kyrgyzstan, where the U.S. and Russia both have air bases, has been plagued by instability and anti-government protests since Bakiyev came to power in 2005 toppling his long-serving predecessor.
The opposition on Saturday accused the authorities of corruption, clampdown on the opposition and free speech, and of "intentionally steering the country toward a crisis to privatize major industrial enterprises."