DUBLIN, Ireland (AP) – More than 500,000 people lined the streets of Dublin to watch Ireland’s major St. Patrick’s Day on Thursday, an extravaganza of artistic skills that featured towering butterflies and teens dressed as mad professors.

The parade, the centerpiece of a five-day festival culminating this weekend with Europe’s biggest annual fireworks show, involved about 3,000 participants in brightly colored costumes and a half-dozen marching bands from Ireland and the United States.

Several floats had to be shortened, or partly deflated, so they could clear the overhead power lines of Dublin’s new light rail network. It cuts across O’Connell Street, the capital’s major thoroughfare, where crowds stood 10 deep to watch the passing spectacle.

Donal Shiels, chief executive of the festival, called the turnout, estimated at more than 500,000 people, “the biggest crowd ever in the history of the parade,” though this couldn’t be independently confirmed.

While the midday parade passed in a jovial atmosphere, police were on high alert for alcohol-fueled trouble at night.

Officials reported 18 arrests on various public-order offenses in central Dublin by Thursday evening. But they said it wasn’t nearly as bad as last year, when police arrested 47 people on St. Patrick’s Day for disorderly behavior, vandalism or assault.

The Dublin lord mayor, Michael Conaghan, who led this year’s parade, said he was relieved that problems associated with public drunkenness improved this year.

Most of Ireland’s leaders missed the Dublin festivities in favor of promoting Irish interests overseas. Prime Minister Bertie Ahern was the guest of President Bush, while President Mary McAleese was touring Japan.

“As our global Irish family and friends celebrate this day through the expression of our culture and heritage in our language, literature, games, poetry, music and dance, I hope that the legacy of St. Patrick will long encourage us to treasure our strong community spirit and tradition of welcome and care for one another,” McAleese said in her traditional message on St. Patrick’s Day, a national holiday in Ireland.

Several other cities and towns across this republic of 3.9 million people mounted their own parades, including in the second-largest city of Cork, this year’s European City of Culture. In Limerick, parade organizers emphasized Ireland’s increasing status as a destination for immigrants from eastern Europe and Africa.