HELSINKI (AP) -- Alexander Stubb was sworn in Friday as Finland's new president and said the Nordic country has taken "the final step into the Western community of values" by becoming a NATO member.

The 55-year-old former prime minister replaced President Sauli Niinistö, who held the job for two six-year terms.

Stubb, a conservative, was elected head of state in a narrow Feb. 11 runoff victory against independent candidate and former Foreign Minister Pekka Haavisto of a left-leaning party.

Stubb has held several government posts, including foreign minister, and led the Finnish government in 2014-2015.

He and Niinistö arrived together in high hats at the 200-seat Eduskunta legislature. Stubb took his oath in Finnish and Swedish, Finland's two official languages, becoming the country's 13th president since it gained its independence from the Russian empire in 1917.

Finland joined NATO in April following Russia's 2022 attack on Ukraine. It has the military alliance's longest land border with Russia -- 1,340 kilometers (830 miles) -- and is one of the most active European providers of military and civilian aid to Ukraine.

Stubb said in a speech that "as a result of allying ourselves militarily and joining NATO we have taken the final step into the Western community of values" to which Finland has belonged "in spirit throughout its independence."

"We will see regionalization of power as well as unholy alliances," Stubb said. "In other words, a kind of an à la carte world, where allies are cherry-picked. We must remain alert."

The president of Finland, a nation of 5.6 million people, holds executive power in formulating foreign and security policy together with the government. The president also commands the military.

Stubb is expected to remain above the fray of day-to-day politics and stay out of domestic political disputes while acting as a moral leader of the nation. The president appoints the prime minister and Cabinet members.

Niinistö said in a speech that "there is concern about the future of NATO," and referred to a comment by former U.S. President Donald Trump, the front-runner for the Republican Party's nomination this year, that he once warned a NATO ally that he "would encourage" Russia "to do whatever the hell they want" to countries in the alliance that don't spend enough on defense.

"The comment 'they must pay their bills' was probably drafted for domestic use," Niinistö said. "It is high time to awaken to securing the state of peace, in other words, to strengthen ourselves."

Speaking to Stubb, Niinistö said, "I wish you strength and wisdom in these unpredictable times."

Until recently a professor at the European University Institute in Florence, Italy, Stubb holds a doctorate in international relations from the London School of Economics.

"Even though the world seems to be in chaos right now, we should not despair," Stubb said. "Fear is the worst possible guiding principle in foreign policy."

Working together globally is necessary, otherwise "we cannot mitigate climate change, solve conflicts, boost the economy, enhance well-being or manage immigration," Stubb said.

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Jan M. Olsen in Copenhagen contributed to this report.