"Only a real dialogue between the authorities and the opposition aimed at finding a lasting political solution based on democratic principles within the constitutional framework offers a way forward," a statement issued from her spokesman's office said.
The British and Foreign Commonwealth Office issued a travel alert Thursday warning of the potential for violence ahead of the Sunday contest.
"Some of these [protests] have been violent, including the use of firearms, and there have been casualties and deaths," the advisory read. "The situation is unpredictable and further protests are expected."
The Bangkok Post reported Thursday so-called Red Shirts, members of the opposition United Front for Democracy, canceled a rally Friday "to avoid unnecessary violence."
Ashton said political conflict in Thailand is an internal matter, though Thai people should be able to exercise their right to vote and to peaceful protest without fear.
Prime Minister Yingluck Shinawatra and her allies are expected to win the election. Opposition groups threatened to boycott the process.
At least 89 people died as a result of political violence in Thailand in 2010.