The self-styled "People's Army" -- a coalition of ultra-royalist groups who loathe the Puea Thai ruling party and its exiled figurehead Thaksin Shinawatra -- has vowed to protest in Bangkok on Sunday, possibly for several days.
It is targeting a government-backed bill, due to enter parliament on August 7, which proposes an amnesty for those involved in several bouts of political violence that have convulsed the nation since a 2006 coup toppled Thaksin.
The Thai government has already invoked a special security law to control the protests and more than 1,600 police were deployed Saturday to protect key government buildings, while thousands more were put on standby.
"As Prime Minister and a Thai citizen I worry that the rally could lead to violence," Yingluck said in a weekly television broadcast, addressing increasing fears of unrest.
In a direct appeal to her opponents -- including the People's Army -- and pro-government groups such as the powerful "Red Shirt" street movement, Yingluck called for talks to diffuse festering political tensions.
"Although there is only a one percent chance of success, I want the conflict to end in this generation," she said.
Thailand has been scored by deep political divisions which have often spilled into violence since the overthrow of Thaksin, a deeply divisive figure who lives in self-exile but still draws loyalty among the kingdom's poor, rural working class.
The contentious amnesty bill would scrap charges against protesters involved in incidents from the September 2006 coup until May 2012 -- barring the leaders.
If passed, its opponents fear Yingluck's government will manipulate the bill to grant her brother an amnesty, paving the way for his return by waiving his two-year jail sentence on corruption charges.
"Our goal is to overthrow the Thaksin regime -- which is the rule by Thaksin for Thaksin, and to replace it with the rule by the people for the people," said People's Army spokesman Thaikorn Polsuwan.
"We come to demonstrate peacefully, unarmed and under the constitution," he added.
On its Facebook page the People's Army said the rally will start on Sunday afternoon at a major park in the city's commercial centre.
Protesters Saturday erected tents outside Lumphini Park, according to Police Major General Piya Uthayo who said the special security law did not extend to the zone.
"Protesters will trickle in and are expected to reach 4-5,000 (over Saturday)," he added.
Police have said they are authorised to use water cannon, tear gas and rubber bullets if violence erupts.
An attempt to introduce an amnesty bill into parliament was aborted last year after "Yellow Shirts" -- who support the opposition Democrat Party -- rallied outside the legislature, evoking memories of their staunch street opposition to Thaksin in 2006.
Two months of protests in 2010 by "Red Shirt" supporters of Thaksin sparked a military crackdown that left about 90 people dead and nearly 1,900 wounded in central Bangkok.