Morales' nearly 14-year rule of Bolivia appeared threatened Sunday as preliminary results by the country's electoral authority said that with 83 percent of the vote counted, Morales of the Movement for Socialism Party led with 45.3 percent to Mesa's 38.2 percent, which is not enough to win the election outright. To win, Morales needed more than 40 percent of the vote and a 10-point lead over the second-leading candidate.
If the results maintain, the run-off will take place on Dec. 15.
The 66-year-old Mesa of the Civic Community Party called the result a "triumph" before his supporters in La Paz.
"This is no ordinary election," he said. "What's at stake is the destiny of Bolivia, what's at stake is the democracy of our country."
However, after the Supreme Electoral Tribunal stopped announcing ongoing ballot counting, he took to Twitter to demand that it continue with the tally.
"The [Supreme Electoral Tribunal] once again fails to meet its commitments," he said. "... We demand that the count be resumed!!! What is happening is very serious."
The Organization of American States asked the electoral tribunal to explain why it stopped announcing the count.
Morales, 59, a former cocoa farmers union leader, reassured his supporters in La Paz that he would win the election once all the rural votes were counted.
"That's why, sisters and brothers, we're going to wait until the last vote is counted to pursue and continue our process of change," he said.
Morales was elected the country's first indigenous president in 2005 after Mesa stepped down. However, Morales has come under increasing scrutiny due to the country's growing deficit and accusations of corruption in the government.