Myanmar was largely cut off from the world for a third night running, after the generals who deposed and detained civilian leader Aung San Suu Kyi on February 1 shut down the internet.

With anti-coup protesters expected to gather again Wednesday in major cities, as they have done for days, the UN envoy, Tom Andrews, sounded the alarm.

"I fear that Wednesday has the potential for violence on a greater scale in Myanmar than we have seen since the illegal takeover of the government on February 1," Andrews said.

Andrews said in a statement that with protesters amassing in the commercial capital Yangon, he had "received reports of soldiers being transported into at least Yangon from outlying regions".

"In the past, such troop movements preceded killings, disappearances, and detentions on a mass scale," he said.

"I am terrified that given the confluence of these two developments -- planned mass protests and troops converging -- we could be on the precipice of the military committing even greater crimes against the people of Myanmar."

Andrews said he also had "word that a secretive trial" of Suu Kyi and deposed president Win Myint had begun this week.

Suu Kyi's lawyer Khin Maung Zaw announced Tuesday that a second charge had been lodged against his client.

He said she and Win Myint were expected to appear via video conference during a March 1 trial.

Andrews called on the international community to put pressure on the generals and "convince the junta that rallies planned for Wednesday must be allowed to proceed without detentions or violence".

"Continued repression of the people of Myanmar's basic liberties and human rights must end immediately," he added.

He also urged foreign businesses to cut ties with Myanmar if the generals "continue down this violent path".

The military justified its power seizure by alleging widespread voter fraud in November elections won by Suu Kyi's party.