SINGAPORE - The door of an LRT train opened while the train was in operation on Friday morning, alarming commuters.

The incident coincided with a five-hour service disruption on the Bukit Panjang LRT from about 5.50am to close to 11am on Friday (Jan 22).

Inner loop Service B of the line was down for that duration, and only service A was in operation. The Straits Times understands that the disruption was due to a "signalling fault".

A photo of the door opening incident was posted on Twitter and Facebook by student Dilla Damitra. It happened at 6.46am when the train was travelling from Bukit Panjang to Senja, she tweeted.

"Thank you for scaring me early in the morning with a sudden jerk leading to this door opening. What a way to start a Friday," Ms Dilla wrote on Facebook.

When contacted, Ms Dilla, 16, told The Straits Times: "I was definitely shocked because I didn't see that coming. Luckily, no one was standing by the door at the time."

Another student, Nurul Nadhirah, 15, posted a second photo of the open door in response to Ms Dilla's tweet.

She said that the train was stuck with the door open for about 20 minutes between Bukit Panjang and Senja stations.

"Everyone stumbled and I could see fear in everyone's eyes, especially on a young girl. She skidded but was quickly pulled by a lady," Ms Nurul said.

"I was scared because... there were a few people standing near the doors and I was afraid they might fall out of the train."

Ms Nurul said the passengers had problems accessing the emergency phone.

"The box was really hard to open and I wanted to give up, but I could see that if I do not give my all we will be stuck there until the next train arrived," she said.

But when they gained access to the phone, it was not functioning, according to both students. Eventually, a woman called SMRT on her mobile phone to report the problem, Ms Dilla said.

An LRT staff member appeared soon after that to check the doors, and drove them to Senja station, Ms Nurul said.

At Senja station, the doors opened and closed repeatedly, and an announcement said that they were at Teck Whye station, Ms Nurul said.

Many passengers alighted at Senja but she continued on the train to Jelapang station where she alighted, she said.

Ms Nurul, who attends West Spring Secondary School, was a few minutes late, but she said her principal was informed that there was an LRT service disruption.

When contacted, SMRT said that their staff reacted quickly to the incident.

"When our staff was alerted to the incident, he quickly boarded the affected train-car to check that passengers were safe, before closing the door and driving the train back to the station. We are investigating the incident," said Mr Patrick Nathan, SMRT vice-president of corporate information and communications.