Thai police arrested a key leader of a protest which began several weeks ago, on Saturday, according to the opposition movement's television channel.
Chamlong Srimuang, a retired general, has been a driving force behind the People's Alliance for Democracy protest.
A court issued arrest warrants for him and eight other protest leaders in late August, the day after thousands of protesters broke into government ministries and a state television station before over-running the prime minister's official compound.
The nine were accused of inciting unrest and trying to overthrow the elected government, charges that amount to treason and can carry the death penalty.
The latest arrest occured after Chamlong Srimuang left the compound, where hundreds of protesters remain behind barricades.