BRUSSELS (REUTERS) - A lion escaped its enclosure at a Belgian zoo on Thursday (June 21) and was shot dead by police after attempts to sedate the animal failed, the zoo said, drawing sharp criticism from animal rights groups and politicians.

The lion managed to free itself from its enclosure at Planckendael Zoo north of Brussels early on Thursday and visitors and staff were immediately evacuated or told to stay indoors.

"After two attempts to sedate the lioness we had to put the animal to sleep with a shot," Planckendael Zoo said on Twitter.

Flemish minister in charge of animal welfare Ben Weyts was critical of the zoo and announced an investigation.

"I think this is horrible and inexplicable," Weyts told state broadcaster VRT. "I don't understand why more attempts were not made to sedate the animal," he added.

Animal rights group Gaia said the attempts to put the animal back into its enclosure smacked of a "cowboy mentality" by the zoo and police.

Planckendael Zoo said in a statement it had to put down the animal as the situation had become too dangerous.

"We did not want for this to happen but unfortunately there was no other choice. It is very sad day for the police officers involved and the zoo," a spokesman for Mechelen-Willebroek police said.