The executions announced Monday sparked condemnation from around the globe, heightened fears that more will follow and prompted calls for sterner international measures against the already-isolated junta.

But the military authorities were defiant, with spokesman Zaw Min Tun insisting the men were "given the right to defend themselves according to court procedure".

"If we compare their sentence with other death penalty cases, they have committed crimes for which they should have been given death sentences many times," he said at a regular press briefing in the capital Naypyidaw.

"They harmed many innocent people. There were many big losses which could not be replaced."

The prisoners, who included a former lawmaker from ousted civilian leader Aung San Suu Kyi's party, had been allowed to meet family members through video conferencing, he said, without providing details.

The country's foreign ministry said it "rejects in the strongest terms" criticism doled out by international powers including the UN, EU, United States and former colonial ruler Britain.

"Such concerns and criticisms on the Myanmar government's legal actions could be tantamount to interfering in the internal affairs of the country and indirectly abetting terrorism," the ministry said in a statement.

- 'Extremely troubled' -

Phyo Zeya Thaw, a former lawmaker from Suu Kyi's National League for Democracy (NLD) was arrested in November and accused of orchestrating several attacks on regime forces, including a gun attack on a commuter train in Yangon that killed five policemen.

He was sentenced to death in January for offences under anti-terrorism laws.

Democracy activist Kyaw Min Yu -- better known as "Jimmy" -- received the same sentence from the military tribunal.

The junta had previously issued an arrest warrant, alleging he had incited unrest with his social media posts.

It would be up to prison authorities to decide whether their families would be permitted to retrieve their bodies, Zaw Min Tun said.

The two other men were sentenced to death for killing a woman they alleged was an informer for the junta in Yangon.

The junta has sentenced dozens of anti-coup activists to death as part of its crackdown on dissent after seizing power last year, but Myanmar had not carried out an execution in decades.

After a chorus of international condemnation on Monday, there was fresh criticism of the junta on Tuesday.

The Association of Southeast Asian Nations (ASEAN) bloc, which has led diplomatic efforts to end the crisis, said it was "extremely troubled and deeply saddened" by the executions.

In a statement issued by current chair Cambodia, it accused the junta of a "gross lack of will" to engage with ASEAN's efforts to facilitate dialogue between the military and its opponents.

In Bangkok, hundreds of people staged a noisy protest outside the Myanmar embassy.

Some held photos of Ko Jimmy and Phyo Zeya Thaw alongside Aung San Suu Kyi as they chanted "We want democracy."

And Malaysia's foreign minister Saifuddin Abdullah slammed the executions, calling them a "crime against humanity".

He called for a review of the so-called five-point consensus agreed by Southeast Asian leaders last year aimed at defusing the political crisis in Myanmar following a coup.

A spokesperson from the foreign affairs ministry of Thailand -- another ASEAN member -- said it regretted "the loss of four lives which aggravates the vexing problems of Myanmar".