Crowds have flocked to one of Kyiv's most beloved squares to see the Ukraine's main Christmas tree lit up to mark Saint Nicholas Day according to a new calendar celebrating Christmas on December 25.

With their military locked in the 21-month-old war against Russia, it was the first time many Ukrainians marked the holiday under the new calendar endorsed by the country's main Orthodox Church to distance itself from practices in Russia.

"Even during these difficult times we have the opportunity to come and see our Christmas tree in our capital’s centre," said homemaker Olesia Polyarosh, 29.

"Maybe not to celebrate, but to feel the Christmas spirit despite everything, that we still can mark it in our country."

A cheer went up as the lights were switched on the tree in the square just outside the 11th century St. Sophia Cathedral, while families sampled pancakes and mulled wine in adjacent kiosks.

Most Ukrainians are Orthodox Christians and the country's main church agreed earlier this year to change away from the Julian calendar under which Christmas is celebrated on January 7.

Most mainly Orthodox countries mark Christmas on December 25, but Russia and Serbia are among those that still celebrate on the later date. 

Many in the square said they agreed with the changed date.

"We should not have anything in common with that country," Polyarosh said. "Nicholas and our boys (servicemen) are holding the country together today. The tree is shining bright, it's not too big, exactly what we need in these times."

Engineer Kateryna Didyk, 32, said it was the first time she had marked St. Nicholas day so early, instead of on December 19, but acknowledged: "Actually, for the last seven years I have been celebrating Christmas on the 25th." 

Didyk said it felt a bit early to celebrate Christmas, but she had bought a tree on Tuesday and started decorating it.

Kyiv mayor Vitali Klitschko symbolically lit the tree, saying private donations had funded it to enable public money to be set aside for the army.