Protesters also set two offices affiliated with two state agencies on fire in the same province, prompting outgoing Interim President Moncef Marzouki to appeal for calm.
Kebili's Sunday Market area witnessed popular protests on Wednesday morning against the results of Sunday's presidential runoff vote, which was won by Nidaa Tounes head Beji Caid Essebsi, eyewitnesses said.
They added that protesters had torched the party's local office, along with a provincial administration building and a security directorate.
The extent of the damage, however, remains unclear.
Eyewitnesses said that civil defense personnel had intervened to extinguish the fire.
Police deployed in the area were forced to withdraw in the face of angry protests, according to witnesses.
"We preferred to pull out of the area to allow the deployment of army troops," a police source told The Anadolu Agency.
"We did not want to complicate matters," the source added on condition of anonymity.
Other eyewitnesses, meanwhile, said the anger was palpable – especially among young people who rejected the results of Sunday's vote.
They said Essebsi's victory jeopardized the goals of Tunisia's 2011 uprising, which ended the autocratic rule of former President Zine El Abidine Ben Ali.
Similar protests erupted on Tuesday in Douz, another city in the Kebili province, but police quickly intervened with teargas to disperse demonstrators.
"I call on residents of Kebili to stop their protests," the Tunisian presidency's Facebook page quoted Marzouki as saying.
He warned that a planned international festival in Douz could be negatively affected by the demonstrations.
Essebsi was declared winner of a hotly-contested presidential run-off vote held Sunday, in which he clinched 55.68 percent of the vote. His rival, Marzouki, won 44.32 percent of the vote.
Sunday's presidential election is seen as the last step in the country's transition to democracy following Ben Ali's ouster in 2011.