Gathered at Bangkok's Victory Monument despite soaring temperatures on Saturday, many sang patriotic songs and listened to speeches denouncing Paetongtarn and her father, Thaksin Shinawatra, also a former prime minister.
The demonstrators voiced their support for the country's army, which has always retained substantial power in the Southeast Asian nation.
Police estimated there were about 2000 protesters by mid-afternoon, though more had been expected to join as the temperature cooled.
Some locals accuse Paetongtarn and her family of allowing the conflict to escalate due to their close ties with Cambodia's former prime minister, Hun Sen.
A court suspended Paetongtarn last month after Hun Sen, still a major power in his own country, leaked a phone call in which she called him "uncle" and appeared to denigrate a Thai general, angering many.
The most recent clashes ended with an uneasy Malaysian-brokered ceasefire late last month.
Some 260,000 people were displaced during the fighting.
"Ung Ing, you need to leave," said one well-known conservative columnist and protester, Jittakorn Bussaba, using Paetongtarn 's nickname.
"Because there's blood on your hands. People have died because of you."
Thaksin and his family should not run or command this country anymore, another protester declared.
"Ung Ing has damaged the country," said 58-year-old Ammorn Khunthong.
There were many familiar faces from a conservative, pro-royalist group once known as the Yellow Shirts, longtime foes of Paetongtarn's father, who was toppled in a military coup in 2006.
Thaksin, a billionaire tycoon, entered politics by founding his own political party and buying the loyalty of local political bosses nationwide.
He was often accused of bullying critics and not separating his business dealings from those of the government.
Yellow Shirts rallies also helped oust the elected government of Thaksin's sister, Yingluck Shinawatra, in a 2014 coup.
The army in Thailand plays a major role in politics and has staged 13 successful coups since the country became a constitutional monarchy in 1932.
