The US has expressed concern over a crackdown in Malaysia on protests last Saturday.
The state department's Mark Toner said people should be allowed to "freely express their democratic aspirations".
Malaysian police cordoned off central Kuala Lumpur on Saturday and used tear gas and water cannon against thousands of people demanding electoral reform.
About 1,650 people were arrested at the rally, but the police said most were released shortly afterwards.
Opposition leader Anwar Ibrahim was one of a small number of people who were injured in the protest. He was taken to hospital after he fell to the pavement after a tear gas attack.
A group calling itself the Bersih 2.0 coalition organised the rally, saying the electoral system is plagued with fraud.
They want longer campaign periods, automatic voter registration and equality of access to the largely government-linked mainstream media.
In his regular news conference, Mr Toner said he had concerns about what had happened on the weekend.
"We stand for... the right for people to freely express their democratic aspirations and express their views freely," he said.
"I would stress that those must be peaceful demonstrations. So we did have some concerns; we continue to monitor the situation closely."
The UN and rights groups have already expressed alarm at the treatment of the protesters.
"The Malaysian government should launch a prompt, impartial, and transparent investigation into the use of excessive force and unwarranted arrests by the security forces," said US-based Human Rights Watch.
British Prime Minister David Cameron is meeting his Malaysian counterpart Najib Razak later, and Amnesty International has urged him to raise the issue.
Malaysian officials have defended their actions, saying the rally was illegal and the force used was proportionate.