But on Wednesday, their stand-off with the government turned violent after security forces opened fire over the head of the demonstrators. One soldier died after being shot in the head and 20 people were injured.
"The government used force to crack down on innocent protesters, and it's likely there will be further violent crackdowns," Jaran Ditsatapichai, a red shirt leader, said on Thursday at the movement's main demonstration site in Bangkok's upscale shopping district.
The red shirts have reinforced roadblocks and stepped up checks on the perimeter of their sprawling protest site, which has been fortified with barricades made from piles of tyres, razor wire and bamboo stakes.
The protesters also presented a letter to a European Union delegation on Thursday urging the bloc to send observers to prevent any further action by the army.
David Lipman, the EU ambassador in Bangkok, met the protesters briefly and called for a peaceful, negotiated solution to the crisis, an EU statement said.
However, Kasit Piromya, Thailand's foreign minister, insisted that the deadlock was a matter for the country's government and said that no outside assistance was needed.
"There is no need for international intervention at this point in time," Kasit told a news conference during a visit to Jakarta.
"We're very much in control of the situation and it's still very much an internal affair of Thailand."
The violence flared on Wednesday when a group of about 2,000 red shirt protesters moved out of the central shopping area they have occupied since April 3, heading to meet supporters in a northern suburb.
Soldiers barred the way and fighting broke out on a crowded highway about 40km north of central Bangkok.
Government troops said they fired rubber-coated steel bullets into a crowd of red shirts as they tried to charge a barricade outside Bangkok, although witnesses said soldiers fired live rounds.
The red shirts hurled stones, shot metal balls from sling-shots and launched fireworks at the cordon of 450 soldiers. Footage captured by an Al Jazeera cameraman appeared to show a red shirt protester holding a handgun.
Medics told Al Jazeera the soldier was killed by a live round to the head and witnesses said he was shot through his helmet while riding on a motorbike towards security forces, apparently caught in friendly fire. Another soldier was among the wounded.
The fighting finally stopped when rain drenched the area.
There was no further violence during the night after the protesters pulled back to the Rachaprasong shopping area and their numbers had dwindled to less than 1,000 by dawn on Thursday.
The red shirts have said they are prepared to enter negotiations with the government, but only via a third party. One red shirt leader told Al Jazeera that talks were already under way with a Thai third party, but did not give more details.
Hopes for a negotiated settlement could be dashed, however, by the return of the yellow shirts, who have said they will take unspecified action themselves if the government fails to "strictly and efficiently enforce the law".
The yellow shirts, representing royalists, the business elite, aristocrats and urban middle class and led by People's Alliance for Democracy (PAD), are opposed to Thaksin Shinawatra, the prime minister ousted in a 2006 coup, who is supported by many red shirts.
The yellow shirts staged a crippling eight-day blockade of Bangkok's airports in December 2008, which left more than 230,000 tourists stranded, disrupted trade and led to credit rating downgrades for Southeast Asia's second-largest economy.
The siege ended when the then ruling party, aligned with Thaksin, was dissolved for electoral fraud, paving the way for Abhisit to assume office after a parliamentary vote the red shirts say was influenced heavily by the military in a "silent coup".