Overnight, some hundred Ukrainians including President Viktor Yushchenko and other top state officials laid wreaths at the monument to the victims of Chernobyl in Kiev and lighted candles during a religious service held for the tragedy, the presidential press service said.
In Slavutich, a small town 50 kilometers (30 miles) away from the wrecked nuclear power station, where most of its personnel live, an overnight vigil was due to be held.
"The Chernobyl catastrophe became planetary and even now continues to take its toll on people's health and the environment," the health ministry said in a statement.
On April 26, 1986, reactor number 4 at Chernobyl nuclear power plant exploded, contaminating large parts of Europe but especially the then-Soviet republics of Ukraine, Belarus and Russia.
Over 25,000 "liquidators", mostly Ukrainians, Russians and Belarussians who worked on the ruined reactor and constructed a concrete sarcophagus enclosing it, lost their lives, according to official figures.
The official UN toll in September 2005 set the number of the accident's victims in Ukraine, Russia and Belarus at 4,000, but the figure had been contested by non-governmental organisations.
Officially, Ukraine alone numbers 2.3 million people qualified as "having suffered from the catastrophe."
Some 4,400 Ukrainians, who had been children or adolescents at the time of the accident, were operated for thyroid cancer, which is the most common consequence of radiation, the health ministry said.
The station, whose last reactor continued to produce electricity, was closed down in December 2000. However, its cracked sarcophagus, which contains some 200 tonnes of radioactive magma made up of nuclear fuel, makes it a continued threat.
The magma is "our worst problem. It is highly radioactive and we do all we can so that rain and snow do not make it into the sarcophagus," the Ukrainian Emergency Situations Minister Volodymyr Shandra said in a statement.
Ukrainian authorities had completed reinforcement of the old concrete sarcophagus, which had been constructed at speed shortly after the catastrophe, but a new steel sarcophagus, which would cover the old one, is yet due to be built.