Ukraine's president has undergone drug and alcohol tests demanded by a comedian who is challenging him in a runoff election this month - passing both, according to preliminary results.

Petro Poroshenko, 53, submitted to the tests at a clinic at Kiev's Olympic stadium in front of cameras yesterday, giving blood, hair and urine samples to four laboratories.

A doctor declared that no alcohol or psychoactive substances were found, but added that a full analysis of the president's tests would take up to three days.

Meanwhile, his challenger Volodymyr Zelenskiy, who plays a teacher-turned-president on a popular television show, took a blood test at a private clinic.

He also passed, but a copy of the results he posted was dated to Tuesday, before he had even called on Mr Poroshenko to undergo testing. The clinic later said an employee had mistakenly entered the wrong date on the form.

The barrage of testing arose from a war of words after Mr Zelenskiy won 30 per cent of the vote in the Ukrainian presidential election on Sunday, trouncing Mr Poroshenko, who had 16 per cent, and former prime minister Yulia Tymoshenko with 13 per cent.

Since no candidate won a majority, Mr Zelenskiy and Mr Poroshenko will face each other in a second round on April 21. The victor will lead a country that last year became the poorest in Europe and continues to wage a simmering conflict with Russia-backed separatists in the east.