"Today I am signing a decree on introducing a state of emergency, which will be adopted the Seimas of the Republic of Lithuania in its extraordinary session," Nauseda told reporters.
"In light of the circumstances, we must take legal measures to strengthen our external security," he said, news agency BNS writes.
The council also decided to ask NATO to invoke Article 4 on joint consultations, according to the president.
Previously on Thursday morning, Lithuania's Prime Minister Ingrida Simonyte said that Russia had effectively declared war on Ukraine.
In the early hours of Thursday, Russian President Vladimir Putin announced a "special military operation" in Ukraine to defend pro-Kremlin separatists and "demilitarize and denazify" its western neighbor.
Updates:
Lithuania has been placed under a state of emergency from 1 p.m. on Thursday through March 10, according to a decree signed by President Gitanas Nauseda.
The decree allows more flexible use of the state reserve and steps up state border protection.
Among other things, the authorities will have the right to check and inspect vehicles, persons and luggage in the border area.
According to the decree, the state of emergency is being declared in light of Russia's large-scale military buildup along Ukraine's borders, in separatist-held regions in the eastern part of the country and in Belarus, as well as Russia's military action against Ukraine.
The document says the situation creates favorable conditions for hybrid attacks and provocations against Lithuania by Russian and Belarusian authorities, especially in the border area.
According to the decree, the situation posed a threat to Lithuania's national security interests and public peace, which requires "the temporary imposition of emergency measures".
The Interior Ministry is responsible "for achieving the objective of the state of emergency". If needed, it can ask for assistance from the Lithuanian Armed Forces.
Under the law, the decree has to be endorsed or repealed by the Seimas. The parliament is convening an emergency sitting at 5 p.m. on Thursday.
