Billionaire tycoon Thaksin then turned to a portrait of Thailand's widely revered, 81-year-old King Bhumibol Adulyadej, and sang a traditional royal song.
"I pledge in front of the portrait of the King and Queen and in front of the Thai people that myself and my family will be loyal to the King and the Chakri dynasty until the last day of my life," he added.
Al Jazeera's Laura Kyle, reporting from Sanam Luang square close to the royal palace, said protesters had begun to gather from early in the morning, with many bussed into the capital from rural areas in Thailand's north and northeast where Thaksin maintains strong support.
|Buddhist monks joined protesters during the march in central Bangkok [AFP]
Vichai Sangkapai, a Bangkok police chief, said that 1,500 uniformed officers had been deployed, but that the rally remained peaceful.
Demonstrators began dispersing after the petition was delivered, witnesses said.
The petition handed in at the gates of the country's Grand Palace was packed in 10 boxes wrapped in red cloth.
"The petition has been submitted to seek royal assistance to stop the people's suffering," Nattawut Saikur, the protest leader, said.
The group said earlier this month that they had amassed five million signatures but on Monday they said a revised count put the number at 3.5 million.
Their political opponents have cast doubt on both figures.
Abhisit Vejjajiva, Thailand's prime minister, who took office in December, warned Thaksin's supporters against signing the petition and said that only Thaksin or his family were allowed to submit it.
The Red Shirts launched the campaign last month following more than three years of confrontations between supporters and Thaksin opponents.
Thaksin's backers forced the cancellation of a major Asian summit in April and then rioted for two days in Bangkok before a crackdown by the army, in which two people were killed and 123 injured.
Rival "Yellow Shirts", viewed as pro-royalists, shut down Bangkok's airports in late 2008, triggering the collapse of the pro-Thaksin government, and ushering Abhisit into power.
Twice-elected Thaksin still enjoys huge support among Thailand's poor, particularly in rural northern parts of the country, but is out of favour with the Bangkok-based elite in the palace, military and establishment.
Thaksin is being tried in absentia on a separate corruption charge relating to $2.2 billion of funds that were frozen by an anti-corruption body soon after the coup.
Abhisit has said Thaksin, who has several passports and divides his time between a number of countries including the United Arab Emirates, must return to Thailand to face justice.
Rivals have accused Thaksin of trying to weaken the monarchy, a charge he strongly denies.