Three activists chained themselves to railings outside the office while several dozen demonstrators chanted "Freedom to the political prisoner," RIA Novosti reported.
They also sought the release of former Interior Minister Yuriy Lutsenko.
The protests followed on an announcement Friday by the state prosecutor general that Tymoshenko was now a suspect in the killing of Yevhen Shcherban, a businessman and member of the Ukrainian Parliament.
Shcherban and his wife were shot dead at an airport in eastern Ukraine in 1996.
If convicted on the charge, Tymoshenko would face life in prison.
She is serving a seven-year jail prison for abusing her authority in signing a gas deal with Russia in 2009.
Lutsenko is serving a two-year sentence for illegal surveillance during an investigation of the poisoning of presidential candidate Victor Yushchenko in 2004. Yushchenko survived and became president.