By Panarat Thepgumpanat
BANGKOK (Reuters) - At least 20,000 "red shirt" protesters massed in Bangkok on Saturday to mark the anniversary of a 10-week rally that plunged Thailand into prolonged violence and political chaos last year.
The protest was one of the biggest since clashes between troops and demonstrators in April and May that killed 91 people, wounded more than 1,800 and sparked widespread arson and rioting in Bangkok and several rural provinces.
The red shirts said they would continue their regular rallies, despite Prime Minister Abhisit Vejjajiva's announcement on Friday that an election -- their key demand last year -- would be held by the first week of July, after parliamentary dissolution in early May.
"As long as there is no democracy, our people will have to keep moving forward," Jatuporn Prompan, a red shirt leader, told Reuters, adding more rallies would be held on March 19 and April 10.
Police estimated at least 22,000 people had joined the rally by early evening. More were expected to arrive later to mark the beginning of a protest last year that shut down swathes of Bangkok's commercial heart and drew 150,000 people at its peak.
The red shirts say justice and democracy remain elusive in Thailand and are demanding the release of more than 100 demonstrators held in jails across the country and a transparent investigation into the deaths of protesters.
They have promised to honor the result of the election, regardless of who wins, if the vote is fair.
The polls are expected to be a close race between the ruling Democrats and the opposition Puea Thai, and analysts say there is a possibility the outcome will be rejected by supporters of both parties, leaving considerable scope for more instability.
The poll will be his first test of popular support after Abhisit's coalition government came to power in late 2008 in a parliamentary vote the opposition says was influenced heavily by the military.
Many opposition figures believe the army top brass might try to intervene in the formation of another coalition, or even stage a coup to prevent Puea Thai, a party allied with the red shirts and ousted former premier Thaksin Shinawatra, from taking power.
A Puea Thai win is seen as a less favorable outcome for investors because of the intense opposition to Thaksin among the army top brass, influential conservatives and "yellow shirt" nationalists. Another proxy of Thaksin could face a rocky time in office.
Economists say a maintenance of the status quo would bode better for political stability and policy continuity, and allow spending plans and infrastructure projects -- often scrapped or delayed when governments change -- to go ahead.
The red shirts gathered on Saturday in Bangkok's old quarter near to where 25 people, mostly protesters, were killed and hundreds wounded on April 10 in a botched attempt by the military to evict them. Hiro Muramoto, a 43-year-old Japanese cameraman working for Reuters, was among those killed.
The incident remains shrouded in mystery and the military insists it was not responsible for any of the deaths or injuries, a claim most independent observers say is highly implausible.
A small group of yellow-shirts, a movement that helped undermine two Thaksin-backed governments and once supported Abhisit, continued its prolonged sit-in on Saturday to protest the government's handling of a border dispute with Cambodia.
They said their rally would continue, regardless of the election.
"Even if we have a new government, it will not fix our problem," spokesman Panthep Puapongpan said.
(Writing by Martin Petty; Editing by Sugita Katyal)