Thaksin's supporters and opponents have battled for power since a 2006 military coup ousted the former prime minister, who was toppled following street protests accusing him of corruption and disrespect for the country's constitutional monarch, King Bhumibol Adulyadej. Thaksin has lived in self-imposed exile for the past five years to avoid a prison sentence on a corruption conviction.
The battle for power has sometimes led to bloodshed on Bangkok's streets. Most recently, about 90 people were killed in 2010 when Thaksin's "Red Shirt" supporters occupied parts of central Bangkok for weeks before the government, led then by the current opposition, sent the military to crack down.
The latest protests have ended two years of relative calm under Yingluck's government.
Yingluck's administration has struggled to contain the demonstrations, which initially started over opposition to a government-backed political amnesty bill that critics said was designed to bring Thaksin home from exile. The Senate rejected the bill earlier this month in a bid to end the protests. But the rallies have gained momentum and leaders have now shifted their target to the more abstract goal of toppling the "Thaksin regime," which is how protesters refer to Yingluck's government.
"Our demonstration will continue until we can get rid of the Thaksin regime," Suthep told the crowd Sunday.
Suthep accused Thaksin of abusing human rights, corruption and interfering with government officials.
"We will tell the government officials to stop serving the Thaksin regime and stand by the people's side instead," Suthep told The Associated Press. "We will also march to tell the media to report news with neutrality, without distortion, and not to serve the government. They must give people the truth."
Special peace-keeping command spokesman police Maj. Gen. Piya Uthayo said intelligence reports indicated that the protesters would also move to surround key Bangkok locations, including the prime minister's office and Parliament.