(Recasts with Nauseda calling victory)

By Andrius Sytas

VILNIUS, May 26 (Reuters) - Lithuanian Gitanas Nauseda announced his re-election in a presidential ballot on Sunday, following a campaign dominated by security concerns in the European Union and NATO member next door to Russia.

The Baltic nation of 2.8 million people has been a staunch ally of Ukraine since Russia's 2022 invasion. Like other countries in the region, it worries it could be Moscow's next target.

Ballots from nearly 90% of polling stations showed Nauseda, 60, winning roughly three quarters of the vote, followed by Prime Minister Ingrida Simonyte, 49, from the ruling centre-right Homeland Union party.

If confirmed by final results, Nauseda's backing in his bid for a second term will be highest in the country since it split from the Soviet Union in 1991.

A former senior economist with Swedish banking group SEB who is not affiliated with any party, Nauseda won the first round of the election on May 12 with 44% of the votes, short of the 50% he needed for an outright victory.

Just over half of Lithuanians believe a Russian attack is possible or even very likely, according to a ELTA/Baltijos Tyrimai poll conducted between February and March. Russia has regularly dismissed concerns that it might attack a NATO member.

Nauseda told jubilant supporters in the capital Vilnius that he will continue working on the country's defence capabilities.

"Lithuanian independence and freedom is like a fragile vessel which we need to cherish and keep from cracking," he said.

Both Nauseda and Simonyte support increasing defence spending to at least 3% of Lithuania's gross domestic product, from the 2.75% planned for this year.

But Nauseda, who is a social conservative, has clashed with Simonyte on other issues, including whether to give a legal recognition to same-sex civil partnerships, which Nauseda opposes.

He has said it would make such unions too similar to marriage, which Lithuania's constitution only allows between a man and a woman.

Simonyte, a former finance minister and a fiscal hawk, said on Thursday that if she won, "the direction for the country - pro-European, pro-Western - would not change".

"But I would like quicker progress, more openness and understanding, larger tolerance to people who are different from us", she said.

Lithuania's president has a semi-executive role, which includes heading the armed forces, chairing the supreme defence and national security policy body and representing the country at EU and NATO summits.

The president sets foreign and security policy in tandem with the government, can veto laws and has a say in the appointment of key officials such as judges, the chief prosecutor, the chief of defence and the head of the central bank. (Reporting by Andrius Sytas; Editing by Andrew Heavens and Tomasz Janowski)