They played the introduction to the Vatican and Italian anthems and the crowd, which numbered at least 50,000, joined in, waving flags from countries around the world.
"I can't explain how happy I am right down," said Ben Canete, a 32-year-old Filipino, jumping up and down in excitement.
Elected on the fifth ballot, Francis was chosen in one of the fastest conclaves in years, remarkable given there was no clear front-runner going into the vote and that the church had been in turmoil following the upheaval unleashed by Pope Benedict XVI's surprise resignation.
A winner must receive 77 votes, or two-thirds of the 115, to be named pope.
For comparison's sake, Benedict was elected on the fourth ballot in 2005 — but he was the clear front-runner going into the vote. Pope John Paul II was elected on the eighth ballot in 1978 to become the first non-Italian pope in 455 years.
Patrizia Rizzo ran down the main boulevard to the piazza with her two children as soon as she heard the news on the car radio. "I parked the car ... and dashed to the square, she said. "It's so exciting, as Romans we had to come."