Bangkok (Agencies): Thai anti-government "red shirts" assembled tens of thousands of supporters to seek the release of detained leaders on Saturday, underlining worsening problems in the divided nation ahead of elections planned this year.

The mainly rural and urban working class demonstrators challenged a special security law to rally at three locations in Bangkok, including the Supreme Court, which is due to make a ruling on whether to grant bail to seven top leaders held on terrorism charges since violent protests last April and May.

Police said about 30,000 people had gathered by early evening in what was the red shirts' fourth big rally this year. It comes ahead of an election to be held before June as announced a deputy Prime Minister.

The poll is unlikely to settle a long-running crisis in Southeast Asia's second-biggest economy and experts say it could lead to more instability if the outcome is disputed by groups for or against Prime Minister Abhisit Vejjajiva's military-backed Democrat Party, which heads the current coalition.

There was a heavy police presence at the rally, during which the red shirts slammed the government over a state probe into the 91 people killed during last year's crippling 10-week protests broken up by the military.