"Some polling stations will be open for advance voting on [Sunday]," the warning read. "There are likely to be protests at polling stations and some of these could turn violent."
The warning from the British Foreign and Commonwealth Office said violent political demonstrations have been ongoing in Thailand at least since early January. The Thai government declared a 60-day state of emergency Tuesday for Bangkok and surrounding areas.
The Wall Street Journal reported Wednesday pro-government leader Kwanchai Praipana, member of the so-called Red Shirt movement, was wounded when protesters fired on his home. At least 31 people were injured in an explosion last week during an anti-government protest in Bangkok.
Prime Minister Yingluck Shinawatra and her allies are expected to win Feb. 2 elections. Opposition groups threatened to boycott the process.
U.S. State Department spokeswoman Marie Harf issued a statement Wednesday saying her government "strongly condemns" the escalating violence in Thailand, a key U.S. ally.
Shinawatra became prime minister after a general election in 2011. Her brother, Thaksin Shinawatra, was ousted by a military coup in 2006 while he was out of the country and still faces criminal charges.
Clashes between supporters of the Red Shirt movement, loyal to the ousted prime minster, and security forces left at least 89 people dead in 2010.