Seven £10,000 fines issued to illegal rave organisers in Leeds
Judith Blake said she had "fully expected" the city to be placed on the Government's coronavirus watchlist following a recent rise in cases.
Ms Blake told BBC Breakfast on Saturday: "We have been monitoring our number every single day and we recognise that the numbers have been creeping up, so we fully expected to be on the list to become an area of concern.
"We feel there is a bit of a complacency coming in. What we are seeing is the numbers are changing, and actually more young people are testing positive and they are spread around the city.
"So whereas we were focusing more on specific communities at the beginning, it is now clear the pattern is changing."
Ms Blake said the council was working closely with the city's universities ahead of students returning, adding that additional testing was being considered.
She said the council was also urging young people to "recognise their own responsibility" in helping to reduce the spread of the virus.
"Unfortunately we have seen a rise in house parties, but we are working with police," Ms Blake said.
"Last weekend we issued, with the police, seven of the £10,000 fines for organisers of illegal raves."
The fines are part of newly introduced legislation aimed at deterring illegal music events.
While organisers can be hit with £10,000 fines, people not wearing masks and participants in unlawful gatherings can be fined starting at £100, doubling for each repeat offence up to a maximum of £3,200.
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