An American Highway Patrol officer was one of three fans arrested for drunk and disorderly conduct at the Wellington Sevens tournament today.
Inspector Terry van Dillen, who is heading the Wellington police operation at the stadium today, said the fan dressed as the officer was arrested after running onto the pitch.
Two others have joined him and police will keep them out of trouble until they sober up, Mr van Dillen said.
They were drunk and annoying other fans, he said.
"Some people who are here to have some fun have paid good money to get in, so we're respecting that, and if someone's going to be an idiot, we'll sort that out."
But in general the crowd were behaving well, he said.
"It's a fantastic atmosphere."
Wellington Free Ambulance spokesman Daniel Paul said they had treated 30 people in the stadium, with only four of those being grossly intoxicated.
"Most have been for minor injuries and dehydration."
Mr Paul said a lot of the injuries were alcohol-related, but the patients were not overly drunk.
One person had been taken to hospital with a fractured ankle, he said.
"The mood is good - compared to last year there are fewer patients," Mr Paul said.
The atmosphere was good and people were pretty relaxed, "and we'd like to keep it that way", he said.
Wellington's biggest party of the year kicked off in brilliant sunshine today in colourful style with rugby sevens fans dressed as everything from wizards to lizards.
The two-day tournament is known for the weird and wonderful costumes of spectators, many of whom packed waterfront bars before kick-off at 1pm.
Extra police and ambulance officers were on duty as about 70,000 fans were expected in the capital for the event - which has become as known for its off-field antics as the action on it.
Wellingtonian Riki Bradley, dressed as a shirtless cowboy, was among hundreds who gathered at the waterfront before heading to the stadium.
He said the Sevens was "pretty much the best weekend ever".
Jessica Higueras and friends were dressed as glamorous gladiators, modelled on Madonna's outfit at the Super Bowl last year.
She said it took three weeks of planning and three days of sewing and gluing.
Aucklander Michael Bell, 27, was with a group of who had imported bulky plush Care Bear outfits for the event.
"We got them in from China. They cost a fair bit, about $300 each."
Almost 100 people were arrested over the Sevens weekend last year, although only 14 were at the stadium.
As well as extra emergency services staff, Wellington City Council volunteers will be handing out water to fans walking to the venue this year.
The council will also trial having food stalls set up in Courtenay Place, which will be closed to traffic from 5pm tomorrow to 6am Sunday.