UN Secretary-General António Guterres has urged the world community to make sure Monday's coup in Myanmar fails.
The reversal of elections is "unacceptable", he said, and coup leaders must be made to understand this is no way to rule the country.
The UN Security Council is discussing a possible statement, but China is expected to block any form of words which condemns the coup.
Elected leader Aung San Suu Kyi was detained when the army seized power.
Police in Myanmar - also known as Burma - later filed several charges against Ms Suu Kyi, who has been remanded in custody until 15 February.
Neither Ms Suu Kyi nor deposed President Win Myint have been heard from since the takeover.
The coup, led by armed forces chief Min Aung Hlaing, has seen the installation of an 11-member junta.
The military, which has declared a year-long state of emergency, sought to justify its action by alleging fraud in last November's elections, which Ms Suu Kyi's National League for Democracy (NLD) won decisively.
Facebook services in Myanmar were disrupted on Thursday amid reports the military had ordered telecom companies to block the social media platform.
The company confirmed the disruptions, urging "authorities to restore connectivity so that people in Myanmar can communicate with their families and friends and access important information".
Over the past days, activists had set up Facebook pages to co-ordinate opposition to the coup.
The UN secretary general called for constitutional order to be re-established in Myanmar. He said he hoped there would be unity in the Security Council on the matter.
"We'll do everything we can to mobilise all the key actors of the international community to put enough pressure on Myanmar to make sure that this coup fails," he said.
"It's absolutely unacceptable to reverse the result of the elections and the will of the people.
"I hope that it'll be possible to make the military in Myanmar understand that this is not the way to rule the country and this is not the way to move forward."
Western countries have condemned the coup unreservedly, but efforts at the Security Council to reach a common position failed as China dissented. China is one of five permanent members with a right of veto in the council.
Beijing has long played a role of protecting the country from international scrutiny, and has warned since the coup that sanctions or international pressure will only make things worse.
Alongside Russia, it has repeatedly protected Myanmar from criticism at the UN over the military crackdown on the Muslim minority Rohingya population.
Suu Kyi's whereabouts unclear
It has been reported that Myanmar's civilian leader Aung San Suu Kyi is being held at her residence in the capital, Nay Pyi Taw.
She faces charges which include breaching import and export laws, and possession of unlawful communication devices.
The accusations are contained in a police document - called a First Initial Report - submitted to a court.
She was remanded in custody "to question witnesses, request evidence and seek legal counsel after questioning the defendant", the document says.
President Win Myint is accused, under the National Disaster Management Law, of meeting supporters in a 220-vehicle motorcade during the election campaign in breach of Covid restrictions.
Aung San Suu Kyi: The basics
- Rose to international prominence in the 1990s as she campaigned to restore democracy in Myanmar during decades of military dictatorship
- Spent nearly 15 years in detention between 1989 and 2010 after organising rallies calling for peaceful democratic reform and free elections
- Awarded Nobel Peace Prize while under house arrest in 1991
- Led her NLD party to victory in Myanmar's first openly contested election in 25 years in 2015
- Reputation tarnished by failure to condemn military campaign which saw more than half a million civilians from Muslim Rohingya minority seek refuge in Bangladesh
Activists call for civil disobedience
There have been few signs of major protest in Myanmar so far. On Tuesday and Wednesday night, drivers honked their horns in the main city, Yangon (also known as Rangoon), and residents banged cooking pots.
The country has appeared mainly calm following the coup, with troops on patrol and a night-time curfew in force.
However, hospitals have seen protests. Many medics have either stopped work, or continued while wearing symbols of defiance to oppose the suppression of Myanmar's short-lived democracy.
The protesters say they are pushing for the release of Ms Suu Kyi.
They are wearing red or black ribbons to signify resistance, and have been pictured giving the three-fingered salute familiar from the Hunger Games films and used by demonstrators last year in Thailand.
Online, many changed their social media profile pictures to the colour red, in support of Ms Suu Kyi's party.
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