A group of Ukrainians led by President Viktor Yushchenko laid a wreath during the night at a monument to the victims of the catastrophe in which a reactor exploded one night in April 1986.
"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 marking the anniversary.
Demonstrators gathered in the centre of the capital Kiev brandishing placards including one reading: "Don't build a new Chernobyl."
"The consequences of the Chernobyl power station accident are huge," said activist Dmitry Khmara. "We are worried that they are again telling us to go along the risky path of developing atomic energy."
In Minsk, capital of the neighbouring republic of Belarus which suffered fallout from Cherobyl, some 2,000 people protested against plans for the country's first nuclear power station.
"No to another Chernobyl," read one placard. "We have two misfortunes, Lukashenko and radiation," read another. Alexander Lukashenko is the authoritarian president of Belarus, much criticised by the opposition and foreign governments for perceived human rights violations.
In Geneva, hundreds of anti-nuclear demonstrators wearing white masks formed a human chain around the headquarters of the World Health Organisation.
The anniversary was also marked by an all-night vigil in a small Ukrainian town called Slavutich, 50 kilometres (30 miles) from Chernobyl, where many of the reactor site's employees lived.
The disaster occurred on April 26, 1986 at 1:23 am local time, when one of the reactors exploded -- contaminating the Soviet states of Ukraine, Russia and Belarus with the fallout also spreading to other parts of Europe.
Over 25,000 people known as "liquidators" -- most of them Ukrainians, Russians and Belarussians -- died getting the accident under control and constructing a concrete shield over the wreckage, according to Ukrainian official figures.
A United Nations toll published in September 2005 set the number of victims at just 4,000, a figure challenged by non-governmental organisations.
In Ukraine alone, 2.3 million people are designated officially as "having suffered from the catastrophe."
Some 4,400 Ukrainians, children or adolescents at the time of the accident, have undergone operations for thyroid cancer, the most common consequence of radiation, the health ministry says.
Chernobyl nuclear power station was finally closed in 2000 after one reactor had continued producing electricity.
But the dead power station remains a threat because the concrete cover laid over 200 tonnes of magma, consisting of radioactive fuel, is cracking.
The magma is "our worst problem. It is highly radioactive and we are doing all we can so that rain and snow do not make it into the sarcophagus," said Ukrainian Emergency Situations Minister Volodymyr Shandra.
Work is in hand to reinforce the seal hurriedly flung over the reactor in the immediate aftermath of the disaster.
Work will also start later this year on a new steel cover due to be in place by 2012.
UN Secretary General Ban Ki-moon on Friday marked the anniversary by pledging UN assistance for the stricken region's renewal.