The annual 4/20 pot-smoking rally, which drew crowds of up to 25,000 to the plaza of the Vancouver Art Gallery on Monday resulted in 64 people being treated in hospital, including teenagers, said health officials.

All the patients were treated at St. Paul's, making for a very busy night in the downtown hospital.

"64 people in the emergency department is a large number, in an already busy emergency department," said Dave Lefebvre, a spokesperson for Providence Health Care.

April 20, widely known in marijuana culture as "4/20," is a day when thousands of people gather for "smoke-ins" in cities across North America.

Crowds at Vancouver's rally appeared larger than past years, swelling to 25,000 late in the day, according to Vancouver Police. Those gathered blocked traffic on West Georgia, Robson, Hornby and Howe streets downtown.

Pot blamed for illnesses among revellers

Most people looking for medical help had conditions related to marijuana consumption, he said.

"Those illnesses ranged from things like nausea and vomiting to things like heart palpitations. Some people came in with a decreased level of consciousness."

Some of the patients had ingested marijuana from cakes, freezies or smoothies and may not have known the exact dose they were receiving, said Lefebvre.

He wouldn't provide specific ages of the patients, but several were teenagers including one not old enough to drive.

The annual 4/20 smoke-in has drawn concern from parents who said not enough was being done to keep teens away from the marijuana rallies.