The police have arrested former Bar Council president Ambiga Sreenevasan in connection with the anti-GST rally.

In a Twitter post, lawyer Latheefa Koya said Seremban MP Anthony Loke and PSM secretary-general S Arutchelvan were also detained.

The trio would be held overnight and a remand order would be sought tomorrow.

Earlier, Ambiga told Malaysiakini that a police officer had informed her lawyer about the possibility of being detained overnight.

"I will be going to the Dang Wangi police headquarters. The police told my lawyer that I will be arrested and held overnight.

"I am shocked by this. It was a peaceful rally," she said.

The gathering in Kuala Lumpur, which brought together 20,000 people to march from various points to Dataran Merdeka and then to KLCC in protest of the new Good and Services Tax (GST), had dispersed at 5.30pm.

Ambiga was also the former co-chairperson of Bersih, a movement which held mammoth street rallies in the past to press for electoral reforms. She is currently the patron of NGO Negaraku.

Ambiga arrived at the police headquarters at 9.40pm, accompanied by lawyers N Surendran, Latheefa and Eric Paulsen.

"(Ambiga will be) investigated under Section 143 (for unlawful assembly). She's not even one of the organisers," Latheefa told reporters.

Ambiga herself did not know what information the police wanted from her.

"I just said a few words in the beginning (of the protest)," said Ambiga, who was one of the speakers at the rally.

'May Day is celebrated worldwide'

Meanwhile, Arutchevelan, who arrived at 10.50pm, said police had initially told him all was well.

"After the protest, I called the Dang Wangi public order chief and he said everything went well. So I'm amazed they made arrests," he told reporters.

He said May Day is celebrated worldwide and therefore it was uncalled for to crack down on the rally.

He added that the Court of Appeal had ruled that protest locations cannot be made a condition for peaceful assemblies.

Loke said he was halfway to Seremban when he was called to Dang Wangi.

"They said they can arrange a police transport from Seremban if I don't come.

"I don't know what's the rush [...] (but) with the huge turn out today I'm sure they're under a lot of pressure," he added.

Probed under Penal Code and Sedition Act

At 1.30am, a police van ferrying Loke and Arutchelvan left the Dang Wangi police headquarters.

Puchong MP Gobind Singh Deo, who represented Loke, said the two were being taken to the police lockup in Jinjang.

"The police are questioning Ambiga tonight, so the other two are being brought to Jinjang.

"The police will apply for remand tomorrow at 9am," he added.

Gobind said all three are being investigated under Section 143 of the Penal Code and the Sedition Act.

Pandan MP Rafizi Ramli has also been called for questioning tomorrow morning.

"I have discussed with the officer in charge and he has agreed that I come tomorrow at 10am . I will go tomorrow at 10am and most probably will be detained like others," said Rafizi.

In an immediate reaction, Human Rights Watch (HRW) deputy director for Asian division Phil Robertson said that HRW stands in solidarity with Ambiga.

"Speaking and attending a rally is not a crime, let her go unconditionally," he tweeted.