Kyrgyzstan opposition leader Urmat Baryktabassov was arrested on Thursday, following clashes between police and demonstrators, dpa reported.
Baryktabassov and his supporters planned to kill members of the government, President Roza Otunbayeva said.
She said the authorities had the situation under control.
Earlier, police used tear gas and stun grenades to disperse hundreds of demonstrators and prevent Baryktabassov and his supporters from entering the capital, Bishkek.
At one checkpoint 20 kilometres from the city the situation escalated, with special forces using armoured personnel carriers and water cannon to stop the protesters, who were carrying sticks and stones.
Hundreds of Baryktabassov's supporters also gathered outside parliament, despite the blockade and demanded the resignation of the interim government.
Tensions have gripped the former Soviet republic since the ouster of president Kurmanbek Bakiyev in April and the ethnic clashes which followed two months later, killing 2,000 people.
But the interim government Thursday denied reports that Bishkek had declared a state of emergency.
Elections are due in the coming months.