Vatican City - Pope Francis ushered in 2015 in typically unpredictable style on Thursday, launching an appeal for an end to war before light-heartedly teasing the many Mexicans in the crowd at St Peter's Square.

Addressing tens of thousands of pilgrims, the Argentinian pontiff marked both the international day of peace and the 2014 centenary of the start of World War I with an impassioned plea for an end to the many conflicts engulfing the planet.

"Let there be no more wars," he said. "Peace is always possible but we have to go and look for it."

"Peace is always possible and prayer is the root of peace. Happy New Year to everyone. Let it be a year of peace in the embrace of the Lord ..."

On that note, the 78-year-old leader of the world's Roman Catholics abruptly switched tone and gave another example of the easy charm that has made him one of the most popular popes of the modern era.

"I see there are lots of Mexicans in the crowd and I salute them," he said. "Oh, they're noisy these Mexicans!"

Earlier, presiding over a Mass in St. Peter's Basilica, Francis had prayed for "peace in our day; peace in hearts, peace in families, peace among the nations.

"The message of the day of peace this year is 'no longer slaves, but brothers and sisters.'"

The dead of World War I were commemorated at St Peter's with 100 chimes of a giant bell that was cast in their memory from the cannons of all the nations involved.

The Maria Dolens bell is located at Rovereto in northern Italy and commemorates the fallen by ringing out 100 times at nightfall every day of the year. Thursday's chimes at midday were relayed to the crowd in St Peter's on giant screens.