Singapore Prime Minister Lee Hsien Loong's official website has been hacked by people claiming to be members of the hacking group Anonymous.
The attack comes a day after Mr Lee said he would track down the group.
He made the comments after a person claiming to be part of the group posted an online video in which he threatened to hack government websites.
Singapore's internet and phone regulator said part of the website had been "compromised" late on Thursday.
"The PMO main website is still working, and we are working to restore the page that has been compromised," the Infocomm Development Authority said in a statement.
The hackers had posted an image of a Guy Fawkes mask - the symbol of the Anonymous group - on the PM's website with the words: "It's great to be Singaporean today."
Under new rules, unveiled by the Media Development Authority earlier this year, sites "that report regularly on issues relating to Singapore and have significant reach among readers" require individual licences.
The video, which was posted last month, had protested against the government's new rules for news websites.
Earlier this week, Mr Lee said his government would "spare no effort to try and track down the culprits".
"If we can find him, we will bring him to justice and he will be dealt with severely. You may think you're anonymous but we will make that extra effort to find out who you are," he added.
Earlier this month, a hacker - dubbed The Messiah and claiming to be a part of Anonymous - hacked the website of Singapore's main paper, The Straits Times.
He had left a message saying the paper's report on the video was misleading.
According to reports, The Messiah also claimed responsibility for hacking the website of the Ang Mo Kio Town Council last week, the municipal branch of the prime minister's district.
Hackers linked to Anonymous have defaced dozens of websites belonging to Australian businesses and Philippine government agencies during the past week.