• Chinese state media said "foreign forces" were behind the protests.
• The government plans to go ahead with the second reading of the extradition bill on Wednesday.
On Sunday, hundreds of thousands of people rallied against the bill which critics fear allows China to target political opponents in the city.
Speaking to reporters on Monday, she insisted the law was necessary and said human rights safeguards were in place.
Chinese state media said "foreign forces" were behind the protests.
Organisers estimate that one million people took part in Sunday's march, however police put the figure at 240,000 at its peak.
If the organisers' estimate is confirmed as correct, it would be the largest demonstration in Hong Kong since the territory was handed over to China by the British in 1997.
Human rights safeguards
On Monday, Hong Kong Chief Executive Carrie Lam said in a press conference the law would in no way erode any of the special freedoms the territory enjoys.
"The bill wasn't initiated by Beijing," Ms Lam said, explaining the law was proposed out of "conscience" and "commitment to Hong Kong".
She also promised legally binding human rights safeguards, and regular reports of implementation of cases to the legislature.
The government plans to go ahead with the second reading of the extradition bill on Wednesday.
Critics of the bill say it would expose Hong Kong residents to China's deeply flawed justice system, and it would lead to further erosion of the city's judicial independence.
Supporters say safeguards are in place to prevent anyone facing religious or political persecution from being extradited to mainland China.
The march was seen as a major rebuke of Ms Lam, who has pushed for the amendments to be passed before July.
'Protesters find it hard to change minds'
Whichever number you believe, this was a huge protest by Hong Kong's standards and this controversial bill has prompted opposition from the most unexpected corners of society.
Two years ago, Carrie Lam ran on a manifesto of "We Connect", vowing to unite a deeply split society after the 79-day Umbrella Movement in 2014.
Ms Lam was never the most popular candidate running to be Hong Kong's leader - and she won largely because of Beijing's blessing.
She has always been considered a capable, experienced bureaucrat who is deft at tackling thorny issues - her moniker is "good fighter". But critics say the city's first female leader is arrogant, elitist and unwilling to listen to the people.
It was also hard to imagine she would fare worse than her predecessor Leung Chun-ying, a pro-Beijing hardliner who was widely disliked. But the banners out on Sunday - many with her face emblazoned on - is an indication of just how personal this protest has got.
The huge turnout is a surprise to many, but Ms Lam's unpopularity will not necessarily taint her political career.
"The Hong Kong government is now mostly, if not only, accountable to the central government and no longer considers public opinion as its source of legitimacy," Samson Yuen, a political scientist at Lingnan University, told the BBC.
By this logic, the protesters will find it hard to change minds at the top.
On Monday, the protests were strongly criticised by an editorial in the state run newspaper China Daily, arguing that "some Hong Kong residents have been hoodwinked by the opposition camp and their foreign allies into supporting the anti-extradition campaign".
The paper argues that "any fair-minded person" would support the "long overdue" bill meant "to plug legal loopholes and prevent Hong Kong from becoming a safe haven for criminals".
After Sunday's mass protests tapered off, violence broke out between some protesters and police. At least three officers and a journalist were injured, according to police.
Hong Kong has entered into extradition agreements with 20 countries, including the UK and the US, but no such agreements have been reached with mainland China despite ongoing negotiations in the past two decades.
Critics have attributed such failures to poor legal protection for defendants under Chinese law.