Venezuela's opposition candidate Henrique Capriles on Sunday warned there was a plan to alter the results in the special election to succeed late president Hugo Chavez.
"We alert the country and the world of the intention to change the choice expressed by the people," Capriles wrote on Twitter after polls began to close in his race against acting President Nicolas Maduro.
Capriles, who lost to Chavez by 11 points in the October presidential election, called on National Electoral Council president Tibisay Lucena to close all polling stations, saying there were attempts to let people vote in places that had already closed.
Vice President Jorge Arreaza swiftly condemned Capriles, telling him on state-run television to "be very careful."
"This is unheard of. Hopefully they hacked Capriles' Twitter account like President Nicolas Maduro was hacked," he said after the account of Chavez's political heir was taken over by hackers.
After voting in the afternoon, Capriles said he wanted a "true democracy" in Venezuela and accused the government of pressuring civil servants to vote for Maduro.