The protesters are supporters of Rysbek Akmatbaev. The Central Election Commission annulled his registration as a candidate on March 30 on grounds he had not lived in Kyrgyzstan for all of the last five years.
Demonstrators in Bishkek are holding signs and chanting for the resignation of Prime Minister Feliks Kulov.
The Interior Ministry has vowed not to allow the protesters to break the law, warning that ministry troops would take harsh measures to maintain order.
Akmatbaev is a reputed crime figure. The seat he was attempting to run for belonged to his brother, Tynychbek, who was killed last year during a prison riot.
(with material from ITAR-TASS, Interfax)