Myanmar's opposition complained on Sunday that ballot sheets had been tampered with in landmark elections in which its leader Aung San Suu Kyi is standing for parliament for the first time.
National League for Democracy spokesman Nyan Win said there had been widespread complaints that wax had been put over the check box for Suu Kyi's party, which could be rubbed off later to cancel the vote.
"This is happening around the country. The election commission is responsible for what is occurring," he said.
"I have sent a complaint letter to the union election commission. If it continues like this it can harm the prestige of the election."
The NLD is contesting 44 of the 45 seats at stake in today's vote -- not enough to threaten the ruling party's majority, but a seat in parliament would give the opposition leader a chance to shape legislation for the first time.
A 2010 general election, won by the military's political proxies, was marred by widespread complaints of cheating and the exclusion of Suu Kyi, who was released from seven straight years of house arrest shortly afterwards.
In the run-up to this Sunday's by-elections, the NLD complained about campaign irregularities, including alleged intimidation of candidates.
Suu Kyi said on Friday that the vote could not be considered "a genuinely free and fair election" but stopped short of announcing a boycott.