By Amy Sawitta Lefevre

BANGKOK (Reuters) - Thailand's "red shirts" turned out in force on Sunday to warn the judiciary they will not stand by if a plan to amend the constitution is rejected, a rewrite critics say is aimed at allowing exiled former premier Thaksin Shinawatra to come home.

The current prime minister is Yingluck Shinawatra, Thaksin's sister. Her administration, including leaders of the red shirt protest group, describe the amendments and related amnesty moves as part of reconciliation plans to end a seven-year crisis.

The Constitutional Court is examining arguments that the government's amendments could threaten Thailand's constitutional monarchy. A final decision is expected in July, which will determine whether the debate can go ahead in August. The present constitution was brought in under a military government in 2007 after Thaksin was ousted in a coup the previous year.

According to police estimates, 35,000 red shirts had gathered at Democracy Monument in central Bangkok by late afternoon, many from Thaksin strongholds in the north and northeast, meeting in a festive atmosphere under light police presence.

A provisional court order this month caused the suspension of parliamentary debate on changes to the constitution, temporarily averting a crisis with all the potential to flare up into another bloody street protest.

"The courts take their orders from the ruling classes. They are an obstacle to true democracy and that is why we are here today - this country still doesn't have true democracy," Somwang Assarasee, a red shirt leader, told Reuters.

"If the court decides the charter cannot be amended, we will not listen. We are prepared to defy the court."

Rallies by the red shirts, mostly drawn from poorer sections of society, and rival anti-Thaksin yellow shirts have frequently spilled over into violence. At least 90 people died and almost 2,000 were injured during a protracted red shirt rally in 2010 that was put down by the military.

He has chosen to live in exile rather than return and serve time in prison after being found guilty of abuse of power.

The red shirts say the latest court order shows complicity between the judiciary and a powerful elite around the royalist establishment and the army. The courts have made decisions on several occasions in recent years that have caused pro-Thaksin governments to fall. Continued...