"We'll see a continuation of the conflict, the standoff remains and the likelihood of more violence could increase."
The Election Commission said it expected legal challenges to be lodged as early as Monday to try to invalidate the poll and attack the legitimacy of the government
Yingluck said the election was a "positive signal", but a lasting solution was needed.
"This election is part of the democratic process," she told reporters. "I hope all sides can help solve each of the country's problems."
(Read more: Has Thailand's government survived the gauntlet?)
The protesters say former telecoms tycoon Thaksin has subverted a fragile democracy with money politics and populism, doling out subsidies, cheap loans and healthcare to woo the poor and guarantee victory for his parties in every election since 2001.
Thaksin's critics also accuse him of disrespecting the monarchy, which he denies.
The anti-Shinawatra demonstrators enjoy broad support from southerners and Bangkok's middle class and are tacitly backed by a royalist oligarchy that sees Thaksin as a corrupt crony capitalist and resents the rapid rise of his political order.
Thaksin has lived abroad since 2008 to avoid a jail term for a graft conviction he says was politically motivated. Critics say Yingluck is merely a stand-in for him.
Thaksin's supporters accuse the military and the establishment, including the judiciary, of colluding over the years to oust his governments. The military, which has staged numerous coups since Thailand became a constitutional monarchy in 1932, overthrew Thaksin in 2006 but has stayed aloof this time.
With several cases against Yingluck and her party taken up by the courts, Thaksin's supporters fear judges might intervene again and they have threatened to stage their own protests if the government is ousted, by the courts or the military.
(Read more: We don't mind if we lose 'fair elections': Abhisit)
Sunday's election was in stark contrast to three years ago, when Yingluck quit her job as a company executive and helped her Puea Thai Party win an election in a landslide, running on the campaign slogan "Thaksin thinks, Puea Thai acts".