A Buddhist monk sprinkles holy water on supporters of former prime minister Thaksin Shinawatra as demonstrations continue in Bangkok on Monday. (Sukree Sukplang/Reuters)

Thailand's prime minister, backed by a formidable military force, rejected an ultimatum to dissolve Parliament on Monday as tens of thousands of red-shirted protesters vowed to splatter the seat of government with their own blood if their demands weren't met.

Prime Minister Abhisit Vejjajiva, the key target of the massive demonstration, earlier said he could not give in to the protesters' demand to dissolve parliament by midday but left room for compromise.

"Asking for the dissolution of Parliament before noon in exchange for a halt to the demonstrations, we all agreed it can't be done," Abhisit said on nationwide television. "However, it doesn't mean the government coalition parties and I won't listen to their ideas."

Thai Prime Minister Abhisit Vejjajiva boards an army helicopter at a military barracks in Bangkok on Monday. (Associated Press)The protesters believe Abhisit took office illegitimately with the help of the military and other parts of the traditional ruling class who were alarmed by former prime minister Thaksin Shinawatra's popularity, particularly among the poor.

Organizers of the demonstrations in the Thai capital said they're requesting that thousands of protesters donate a small amount of blood, with the aim of collecting roughly 1,000 litres of blood.

"The blood will be taken from the body and democratic soul of the Red Shirts," said a protest leader, Natthawut Saikua, referring to the popular name for the protesters. He said they would start recruiting medical staff for the blood drive Tuesday morning.

They threatened to pour the blood on Government House if their renewed demand was rejected by their extended deadline of Tuesday evening.

A Red Cross official expressed concern over the hygiene of drawing blood from so many people and noted that such a large quantity "could save a lot of lives," if it weren't destined to be spilled.

Protest turns violent

In the first reported violence of the protests, two soldiers were wounded when four grenades exploded inside the compound of the 1st Infantry Regiment, known as the King's Own Bodyguard, said army spokesman Col. Sansern Kaewkamnerd.

He did not blame the demonstrators, who were not in the area, but said there has been intelligence that some elements had been planning such attacks. The grenades were fired from an M-79 launcher, he said.

A force of more than 50,000 soldiers, police and other security personnel has been mobilized in the capital area to deal with the protesters, who are formally grouped as the United Front for Democracy against Dictatorship. They are demanding Abhisit dissolve parliament and call new elections, which they believe will restore their political allies to power.

The Red Shirts' last major protest in Bangkok last April left two people dead, more than 120 people injured and buses burned on major thoroughfares before the army quashed the unrest.

Thailand has been in constant political turmoil since early 2006, when anti-Thaksin demonstrations began. In 2008, when Thaksin's political allies came back to power for a year, his opponents occupied the prime minister's office compound for three months and seized Bangkok's two airports for a week.

With files from CBC News