For over two months, political tensions in Hong Kong have escalated over a proposed extradition bill that would allow those arrested in the territory to be sent to mainland China for trial. Hong Kong citizens are concerned that their civil rights could be slowly eroded under Beijing.
"You know, you could say what you said, but you could also say that he has allowed that to go on for a long time, and, you know, it's been relatively — I think it's been relatively non-violent," said Trump, according to a White House transcript. He was speaking to reporters at the White House before a meeting with Pakistani Prime Minister Imran Khan.
Trump added that "China could stop them if they wanted."
"I think President Xi of China has acted responsibly. Very responsibly. They've been out there protesting for a long time," Trump said. "I hope that President Xi will do the right thing."
The protests in Hong Kong have turned violent at times and is the most serious crisis since the former British colony returned to Chinese rule in 1997.
On Sunday, protesters surrounded China's main representative office in Hong Kong and defaced walls and signs. They also clashed with police.
Later the same day, men in white t-shirts — some armed with clubs — flooded the rural Yuen Long station, and stormed a train, attacking passengers with pipes, poles and other objects, according to video footage.
China has so far not directly intervened in the crisis, but its state media denounced the vandalism at Beijing's Central Government Liaison Office in Hong Kong on Sunday.
"Such vicious acts have seriously trampled on the rule of law in Hong Kong, blatantly challenged the authority of the central government and touched the bottom line of the principle of 'one country, two systems,' which are absolutely intolerable," said state news agency Xinhua in a commentary on Monday.
The Chinese Communist Party flagship paper also said in a front page editorial that the act openly challenged the authority of the central government.
The "behaviors of some radical protesters" tested Beijing's limits and "have crossed the bottom line of the 'one country, two systems' principle," added Chinese foreign ministry spokesman Geng Shuang on Monday.
Hong Kong's Beijing-backed leader, Carrie Lam, also condemned the attack and said it was a "challenge" to national sovereignty.
— CNBC's Grace Shao and Reuters contributed to this report.