Finland became NATO's 31st member in a flag-raising ceremony at the alliance's Brussels headquarters attended by Finnish President Sauli Niinisto just over a year after Russian troops rolled into Ukraine.

"I feel it's a good thing that Finland is joining NATO. We have been here next to Russia for ages," said Outi Lehtimaki, 59, a designer in Helsinki. "My father was in the war with the Russians so this is like a personal thing to me."

Senja Veihlanen, a 25-year-old baker said, "In some ways I think it will make Finland a safer place but then again we don't know what Russia will do. It's a big question for me."

In Virolahti, near the Russian border due east of Helsinki, retired Finnish combat engineer Ilkka Lansivaara had hung his own NATO flag from the side of his house.

"It's a special day for Finland," said Lansivaara, 70, a former soldier whose father was an air force pilot during World War Two. "Now we have power also behind us, not just our own forces," he added. 

Matti Seppala, 78, a retired warehouse worker who grew up near the border said joining would bolster Finland's security even if he was not scared of Russian "sabre rattling".

"I supported joining, you never know with the big brother, what they will do," he said.

In Vaalimaa, once a busy crossing point for Finns and Russians almost midway between Helsinki and St Petersburg, border guard Markus Haapasaari watched the now just slow trickle of traffic. Finland's new status hadn't sunk in for him yet, he said.

The Ukraine invasion, which Moscow calls a "special military operation, has led both Finland and Sweden to abandon decades of military non-alignment and seek safety in the NATO camp.

The accession of Finland with its 1,300-km (810-mile) border with Russia roughly doubles NATO's land frontier facing Moscow at a time when East-West relations have hit their lowest point in decades.

Across the border in St Petersburg, a Russian resident of the city who gave his name only as Nikolai said, "Finland is making problems for itself by joining (NATO)... we used to consider it a brotherly country of the capitalist world, the closest to us in spirit, in relations, in mutually-beneficial economic relations. But now we will consider it as a state that is unfriendly to us."

Ruled by czarist Russia for more than a century, Finland gained independence in 1917. It then desperately fended off a Soviet invasion in 1939 and for a time sided with Nazi Germany in a bid to win back lost territory.

As the war ended with Allied victory, Finland found itself compelled to spend decades maintaining friendly and accommodating relations with its eastern neighbour and treading a sometimes precarious path of neutrality through the Cold War.

Memories of Finland's close relations with Moscow to preserve independence - a tactic known as "Finlandisation" - run deep for many Finns.

U.S. President Joe Biden has remarked how the war in Ukraine, rather than spur a "Finlandisation" of NATO, brought on a "NATO-isation" of Finland.

Finland brings a sizable, well-trained military into NATO and Russia has said it will have to take "counter-measures" to ensure Russian security in response.

Meanwhile, Finland's close partner Sweden continues to wait for ratification of its NATO membership bid in the face of opposition from Turkey and Hungary.

(Reporting by Essi Lehto and Tom Little; writing by Niklas Pollard; editing by Alexandra Hudson)

By Essi Lehto and Tom Little