"Put the interests of the Burundian people before your personal interests and withdraw your presidential bid, because it violates the constitution," Gervais Rufyikiri said in a letter addressed to President Pierre Nkurunziza.
He told France 24 television he had sought refuge in Belgium after opposing Nkurunziza's bid for a third term in power.
"I left... Because I was not able to continue to support the attitude of the president, his desire to lead the people of Burundi on the path of illegality," he told the broadcaster late yesterday from Belgium.
The troubled central African nation has been in crisis since late April over Nkurunziza's controversial bid to stand for another five-year term.
This move is branded by opponents as unconstitutional and a violation of a peace deal that paved the way to end 13 years of civil war in 2006.
Parliamentary elections are due to be held on Monday, ahead of the presidential vote on July 15.