"We are likely to have negative economic growth for the full year in 2019," the South China Morning Post quoted Lam as saying in a briefing.
"To get to the root of the problems, we must stop violence and let calmness restore in the city... But it was unfortunate that on Sunday and last night, rioters found some excuses, and engaged in extreme violence."
She said the city could miss the government's revised growth forecast of anywhere between 0 and 1 per cent this year, down from the 3 per cent growth seen last year.
Third quarter figures for Hong Kong's economy were due on Thursday.
To combat the slowdown, she said the city has so far injected more than HK$20 billion ($2.6 billion) to support the economy including the transport, tourism and retail sectors.
Financial Secretary Paul Chan Mo-po had warned on Sunday that he "would not rule out the possibility" that Hong Kong's GDP for the full year would see an overall decline.
His comments came hours before protesters hurled petrol bombs at police and vandalised shops in what was the 21st straight weekend of anti-government protests.
Clashes between protesters and police also broke out on Monday night over a mysterious smell, which triggered accusations that police were testing chemicals at a facility.
The confrontation ended with vandals targeting shops and police firing tear gas.
The protests have also led to a sharp decline in tourist arrivals, with visitor numbers expected to have slumped 50 per cent in October, the BBC said in a report.
Hong Kong was one of the world's most visited cities last year, with 30 million tourists.
( With inputs from IANS )