By Martin Petty

BANGKOK (Reuters) - Thousands of Thai anti-government "red shirts" were planning a return to Bangkok's commercial heart Friday.

The "red shirts," who will hold a one-day protest they insist will be peaceful, remain a potent extra-parliamentary force, six months after a military crackdown ended a protracted standoff that paralyzed central Bangkok for weeks.

WHY ARE THEY DEMONSTRATING?

The movement, made up mostly of urban and rural working-class Thais, believes the current government is illegitimate and that its backers, the royalist establishment elite and military, have undermined democracy and the judiciary, ensuring the dissolution of parties loyal to former premier Thaksin Shinawatra, ousted in a coup in 2006.

The focus of their 10-week protest from mid-March to May was to demand an immediate election. That was not met and the protests spiraled into rioting, bloody clashes with troops and urban violence that killed 91 people and wounded at least 1,800. More than 30 commercial buildings were damaged by arson.

The red shirts are gathering frequently and calling for those behind the 91 deaths to be brought to justice. They are also protesting the detention of their leaders and other members of the movement. The "red shirts" say at least 400 people have been detained under emergency laws since the rally ended. Prison officials estimate at least 150 are still being held.

ARE THE RED SHIRTS REGROUPING?

It appears they are, to a limited extent. Even with the movement's leaders in detention, demonstrators are still gathering in Bangkok and other provinces, with at least 10,000 massing in the capital on September 19 and 8,000 on October 10 and 12,000 in Ayutthaya on October 17.

Analysts believe anti-government sentiment could become more entrenched as a result of the crackdowns, media and internet censorship, arbitrary arrests and detention without charge.

A government reconciliation plan is far from inclusive and its failure to properly investigate the deaths of protesters while pushing the claim that mysterious "terrorists" were behind most of the killings could add fuel to the red shirts' claims of injustice, with impunity for troops.

The promotion to army chief of General Prayuth Chan-ocha, a staunch royalist involved in the coup that toppled Thaksin and crackdowns on the red shirts in 2009 and 2010, is seen as a statement of zero tolerance toward the red shirts by the government and military. He has said the protests have embarrassed the country.

SHOULD INVESTORS BE CONCERNED?

Investors don't seem too worried for now and continue to pile into Thailand's financial markets, helping lift the baht about 12 percent to a 13-year high against the dollar this year.

Thailand's economic fundamentals remain sound and the government is expecting growth of as much as 8 percent or more this year, despite concerns about the impact of the baht's strength on the $264 billion export-driven economy.

But although peace prevails for now, the country remains deeply polarized and the crisis is far from over.

Neither the red shirts nor the government and its establishment and military allies appear to have softened their stance and the protesters' grievances have not been addressed. There is no indication of an election any time soon.

COULD RADICALS EMERGE?

That's the big worry. Bangkok and surrounding areas have been hit by many mysterious, mostly minor bombings since the protests ended on May 19. With few arrests and scant evidence, the reliability of surveillance and intelligence is being questioned.

On October 5, a massive blast in a suburban apartment killed four people and injured nine. An anti-government activist was among the dead. He had rented the apartment, which was packed with explosives and bomb-making material.

The Department of Special Investigation, Thailand's equivalent of the U.S. FBI, says it has detained red shirts who were plotting plot to assassinate Thai ministers. No evidence was provided but the DSI said the men were indoctrinated with republican propaganda and given training in Cambodia.

Analysts say the movement's lack of leadership is dangerous: with no one to keep them in line, radicals and hard-core splinter wings could emerge.

Another cause for concern is the theft of dozens of rocket-propelled grenades from a military armory. Details from the army remain hazy but little progress seems to have been made in investigating how they went missing and who might have them.

(Editing by Jason Szep)