Prosecutors have filed an appeal against a district judge's decision to sentence an undergraduate to supervised probation for molesting a woman.

Law Minister K. Shanmugam revealed this yesterday, two days after the verdict, adding that he was surprised by the ruling.

Mr Shanmugam said officers in the Attorney-General's Chambers (AGC) disagreed with the verdict and intended to appeal.

"That is consistent with my views as well," he added.

An AGC spokesman said yesterday that the prosecution "has filed a notice of appeal to the High Court with respect to the sentence imposed on Terence Siow Kai Yuan".

"We are unable to comment further as the matter is now before the courts," said the spokesman.

In a Facebook post yesterday, the minister said he could understand how the victim and her parents must feel, adding that the victim's father had written to him.

The 28-year-old victim, who had identified herself openly on Facebook and in an interview with The New Paper, had said she was disappointed with the verdict as she had hoped the perpetrator would be given jail time instead.

Siow, 23, a National University of Singapore undergraduate, had pleaded guilty to one count of outraging the modesty of the woman on an MRT train. Another two similar charges were taken into consideration during sentencing.

The offences were committed in September last year. Sitting next to the woman on the train, Siow touched the woman's thigh twice. When she moved away and alighted from the train, he followed her and touched her buttocks over her clothes while they were on the escalator.

He was sentenced on Wednesday to 21 months of supervised probation. Prosecutors had objected to probation and asked for six weeks in jail, but this was denied by District Judge Jasvender Kaur.

She found that Siow had an inability to control his urges, and described the acts as "minor" in the nature of intrusion.

Citing reasons such as his academic results and the recommendation for probation, she said there "can be no doubt that there is extremely strong propensity to reform". His academic results show that he has the "potential to excel in life", said the judge.

The case has seen strong reactions online, with many saying that the sentence was inadequate and querying why Siow's academic results were relevant to it.

An online petition rallying people to stand against the purported favouritism towards "educated sex offenders" had gathered more than 29,000 signatures as at 10pm last night on the change.org website.

However, lawyers noted that many factors would be taken into account during sentencing, and not just a person's educational qualifications.

Sentencing is a fact-specific exercise, so it is difficult to comment on the judge's decision when there is insufficient information in the public domain, experts told The Straits Times.

In his Facebook post, Mr Shanmugam said that people are entitled to express their views, including their unhappiness with the verdict and their feelings that the punishment is inadequate.

But he added that people should avoid casting aspersions on judges.

"They are doing their duty, to the best of their abilities," he said.

"If, after the appeal is decided, we, as a society, still generally believe that the law should deliver a different outcome, then it is not the courts' fault," the minister added.

"It is then for Parliament to deal with that, change the law. And people know - in Singapore, the Government will move, and put legislation before Parliament to consider."