BANGKOK — At least six people were injured Saturday in a prolonged daylight gun battle between protesters seeking to block the distribution of ballots in Bangkok and would-be voters demanding that protesters cease their attempts to obstruct national elections on Sunday.

After three months of a provocative campaign by protesters to overthrow the government of Prime Minister Yingluck Shinawatra, the clash on Saturday appeared to crystallize the power struggle that has been playing out on the streets of Bangkok.

Ignoring pleas by the United States and the European Union to respect the democratic process — and stoking the anger of many Thais eager to vote — the protesters have blocked the distribution of ballots in parts of Bangkok and southern Thailand, a stronghold of the opposition.

The shooting on Saturday raised fears about further violence as polls opened on Sunday. There were early indications that several hundred polling stations could not open because they lacked ballots or protesters had stopped voters from entering, The Associated Press reported.