Thousands of opposition protesters have gathered in the Kyrgyz capital Bishkek for a rally to demand the resignation of President Kurmanbek Bakiyev.
The rally in the centre of Bishkek has so far been peaceful
Some have erected tents in the main square and say they will remain there until their demands are met.
They are angry at what they say is the president's failure to introduce democratic reforms he promised when he came to power a year-and-a-half ago.
Mr Bakiyev has rejected their calls for immediate constitutional reforms.
Opposition supporters began gathering from early morning, with shops shut amid fears of possible violence and looting.
Carrying placards - and with some on horseback - the protesters marched on the main square outside the presidential building, watched by hundreds of police.
Police said some 5,000 people had gathered, although correspondents said the figure was at least double that.
"We will stay on the square as long as the constitution is not changed," opposition leader Temir Sariyev told the crowd. He said some 300 tents would be set up in the area.
Protesters told the BBC News website that while the atmosphere was peaceful they feared events could turn violent.
"There are many different groups gathering... It feels very risky to be out," shop owner Bakyt Tulparov said.
TULIP REVOLUTION WILTS
March 2005 - Former President Akayev ousted in popular protest
July - Kurmanbek Bakiyev elected President
April 2006 - Thousands protest for end to corruption and crime
November - Opposition calls for constitutional reform and curbing of presidential powers
The BBC's Natalia Antelava says the scene is similar to events in March 2005 when thousands protested in the same square during the so-called Tulip Revolution that drove former President Askar Akayev from power.
The rally comes after weeks of political crisis and several rounds of failed negotiations over constitutional reform.
Opposition leaders accuse the president of breaking a pledge he made earlier in the week to hand more of his powers to the parliament and cabinet.
They say he was to submit the constitutional reforms to parliament on Thursday, which would have meant the immediate dissolution of the current cabinet allowing parliament to choose a new one.
"He chose the path of conflict, and the people's only demand will be his resignation," Melis Eshimkhanov, from the For Reforms opposition group, told the Associated Press.
But in a short speech to parliament, Mr Bakiyev said any reforms should not be adopted in an "hour or two" and called on the opposition not to "exert pressure" on him.
"I'm not avoiding you, but I have to listen to other lawmakers," he told them. "If you exert pressure through rallies, you will get an adequate response."
Kurmanbek Bakiyev won a landslide victory following Mr Akayev's departure, and initially said he favoured reducing the powers of the president and transferring them to the legislature.
But most of his former allies have since turned against him, accusing him of failing to tackle corruption and create a government based on democratic principles.
Any instability in the country could have wider repercussions, our correspondent says.
The only US military base in the former Soviet Union lies just outside Bishkek and is a crucial operation for supplying Nato troops to Afghanistan.