(CNN) -- Ukraine on Tuesday marked the 25th anniversary of the world's worst nuclear disaster with a series of memorials and commemorations at Chernobyl.
The presidents of Ukraine and Russia, Viktor Yanukovych and Dmitry Medvedev, took part in a joint visit to the now-defunct plant.
The pair used the visit to raise concerns about nuclear safety around the world -- a subject at the forefront of many people's minds following the threat to Japan's nuclear industry from the recent earthquake and tsunami.
"We are marking a tragic date," news agency AFP reported Ukraine's Yanukovych as saying. "Twenty five years have passed and we have understood that nuclear accidents have colossal consequences for the population.
"The world has understood that such catastrophes cannot be fought by one country on its own."
Russia's Medvedev called for a new set of international standards to help prevent further nuclear accidents from taking place.
But CNN's Matthew Chance reports that, 25 years on, there are still serious safety concerns about the threat of radiation from the site.
A concrete, steel and lead sarcophagus built over reactor four, where the deadly blast occurred, is now decaying.
A new cover is needed to replace the cracked and rusting one constructed in the months following the disaster, and to protect the fuel rods inside.
The anniversary events began at 1:23 a.m. (22:23 GMT Monday) -- the exact time of the blast -- when Patriarch Kirill, the head of the Russian Orthodox Church, led a memorial service in the Ukrainian capital, Kiev.
A bell tolled 25 times to mark the years that have gone by since the accident.
Mourners also gathered at the Chernobyl victims' monument in Slavutich, 30 miles (50km) away, to lay flowers and light candles in memory of those killed. Many of those who worked at the ill-fated plant lived in the town.
An explosion in reactor four in the early hours of April 26, 1986, left a plume of radioactive debris hanging over the region.
The International Atomic Energy Agency and the World Health Organization say 28 emergency workers died of radiation sickness in the immediate aftermath of the disaster.
About 20 more who received high doses of radiation died of various causes in the following years, and as many as 4,000 cancer deaths are expected as a result of the disaster, according to those U.N. agencies.
Fallout from the nuclear blast was carried across the then-Soviet Union and Europe.
The Chernobyl anniversary comes amid renewed concerns over the safety of nuclear power, as Japan struggles to bring the disaster at its Fukushima Daiichi plant under control.
An earthquake and tsunami hit Japan on March 11, damaging several reactors at the power station, and leading to a number of radiation leaks.
CNN's Matthew Chance said the latest accident had led to a growing tide of public opinion against nuclear power in some countries, including Germany, where protests were held at the weekend.
But he said other, poorer countries with large power needs were dependent on cheap nuclear power, and had nothing to replace it with.
CNN's Matthew Chance contributed to this report.