Sirens blared and public television broadcast warnings instructing citizens to "close doors and windows, seal ventilation systems" and head for the basement. Residents also received text messages on their mobile phones.

Lithuanian officials organised the exercise in reaction to growing public concern over a new nuclear facility in the northwestern Belarusian town of Ostrovets, located just 50 kilometres (30 miles) from the Baltic EU state's capital Vilnius.

The nuclear plant, approved by strongman Alexander Lukashenko's government in 2008 and spearheaded by the Russian state energy corporation Rosatom, is entering its final stages.

Lithuanian President Gitanas Nauseda warned that the facility fails to meet international safety standards and "poses a threat to Lithuanian national security", something that Belarus and Rosatom have repeatedly denied.

"The drill is already providing results and we can see some gaps," Interior Minister Rita Tamasuniene told reporters in Vilnius, advising people to "stay calm but vigilant".

Lithuania would invest in more sophisticated equipment to monitor radiation levels, she added.