Officers from three districts were called to close streets, clear the waterfront promenade and disperse the crowd. One officer used a Taser to subdue an unruly 16-year-old, according to police.
Messages from witnesses on Twitter described melees stretching from The Block to the light rail on Howard Street, with reports of teens beating on a rail car window to get at people inside.
"People from different neighborhoods having beefs," is how the Baltimore Police Department's chief spokesman, Anthony Guglielmi, described the situation. He said the juveniles were arrested on charges of assault and disorderly conduct.
Police used video surveillance to track the youths as they converged on the harbor and surrounding streets. About 9 p.m., police said, two groups of teens started fighting at East Baltimore and Light streets.
Guglielmi said the fracas involved youths from the east and west sides, and that many involved got more combative as police officers waded in to the group. The spokesman said that an officer used his Taser to subdue one young suspect.
About 15 minutes later, Guglielmi said, a 17-year-old girl was stabbed in the 100 block of S. Calvert St. The spokesman said the victim was walking with friends when a young woman ran up to her from behind and said, "Hey, you in glasses," and punched her in the head when she turned around.
Police said the woman then stabbed the victim in her upper arm. No arrest has been made, but police detectives believe the victim and assailant attend the same school.
Elsewhere in the city, officials said unexpectedly large crowds filled the bar and entertainment districts in Federal Hill — where urinating in public and numerous fistfights were reported — and in Canton, where revelers left behind a heap of trash and empty beer cases strewn about O'Donnell Square.
Dustin Ritter, who lives in Canton, said he repeatedly complained to police about public urination and drinking.
"The Southeast District police officers working the event failed to live up to their sworn duties," Ritter wrote in an email to city officials. "Cases upon cases of beer were being consumed in the Square throughout the day. … At times, the line outside of the liquor store to buy cases and 12 packs was 30-40 people deep."
Stephan Fogleman, chairman of the city liquor board, who lives in Canton, said a years-old agreement between police and taverns allows bars to put up tents on St. Patrick's Day. But he said there appeared to be a misunderstanding with police about whether that meant people could openly drink outside in O'Donnell Square.
Officials said the beer came from a nearby corner liquor store, not the many taverns lining the square.
"The square looked more like the Preakness Infield than it did a residential neighborhood," said Fogleman, adding that he watched people walk by his house "with open containers of alcohol every 60 seconds."
Baltimore Police Maj. Bill Davis, head of the Southeastern District, said that there was no miscommunication and that police did not abdicate their responsibilities. "Drinking is not allowed on the square," Davis said, adding that his officers issued several citations.
The major attributed the rowdiness to the unusually hot weather and the holiday falling on the weekend. He said police based deployment on last year's St. Patrick's Day, when it was a Friday. This year, he said, "the crowds were a little more than we expected."
Guglielmi said officers will deploy better next year. "There is no question the neighborhood was destroyed, and it shouldn't have been," he said. "It looked like a garbage dump. Homeowners deserve better."