Prime Minister Yingluck Shinawatra is due to defend herself on Monday at the National Anti-Corruption Commission against charges that she failed to stop corruption and stem huge losses in the government's flagship rice-buying program.
Yingluck's supporters have been protesting at the commission's compound for several days and camping out in front of it.
No one was injured from the grenades, which were thrown into the compound late yesterday, deputy national police chief Gen Ake Angsananont told reporters. A building was slightly damaged at the compound in Nonthaburi province, outside Bangkok.
It was the second grenade thrown at the NACC this week, and the latest in a string of apparently political related grenade attacks that have included targets such as the Criminal Court, the attorney general's office and the home of a high-profile judge.
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The attack came hours after the NACC rejected Yingluck's request to extend the Monday deadline for her defence. It had already granted an initial 15-day extension.
The case comes after several months of anti-government protests against Yingluck to demand her resignation on grounds that she is corrupt and is serving as a proxy for her brother, former Prime Minister Thaksin Shinawatra. The protests have eased in recent weeks but are set to be revived Saturday for a mass rally in Bangkok.