The legal changes were announced as armoured vehicles appeared on the streets of several cities.
The military leader said, “Long sentences and fines will also apply to those found to incite “hatred or contempt” towards the coup leaders.”
Hundreds of thousands of people are taking part in protests every day.
The demonstrators are demanding the release of their elected democratic leaders, including Aung San Suu Kyi, and the restoration of democracy in Myanmar, also known as Burma.
On Monday, Ms Suu Kyi’s lawyer said she would be detained for a further two days. She will then be tried via video link at a court in the capital Nay Pyi Taw on Wednesday, Khin Maung Zaw added.
Ms Suu Kyi was detained with other members of the government on 1 February, but her detention was due to end on 15 February, according to Reuters news agency.
The charges against her include possession of unlawful communication devices – walkie-talkies used by her security staff.
Her democratic party was elected in a resounding victory in November 2020 elections. But the military has alleged voter fraud without providing proof.
What are the signs of a crackdown?
The military’s heightened presence is the latest sign of a potential crackdown on opposition to the coup.
The ruling junta has also announced a series of legal changes, imposing long prison sentences and fines on anyone found to incite hatred towards the military, “by words, either spoken or written, or by signs, or by visible representation”.
Military posted a statement on their website, which said that people preventing the security forces from carrying out their duties could face seven years in jail. While those found to stir up fear or unrest in public could be imprisoned for three years.
The news came hours after the internet was restored. Telecoms operators said they had been told to shut off services from 01:00 to 09:00 local time, Sunday into Monday (18:30 to 02:30 GMT).
Across the country on Sunday, hundreds of thousands of protesters rallied against the military for the ninth day in a row.
In the city of Myitkyina, in Kachin state, shooting could be heard as security forces clashed with anti-coup demonstrators. It was not clear whether rubber bullets or live rounds were being fired.
Five journalists were among those arrested, but were later released.
In Yangon, armoured vehicles were seen on the streets for the first time since the coup. Monks and engineers led a rally there, while motorcyclists drove through the streets of Nay Pyi Taw.
“I’m still worrying because they made a curfew statement… not to go outside between 20:00 and 04:00, but this makes a time for the police and soldiers to arrest people like us,” said the doctor, who cannot be named for safety reasons.
“The previous day they [broke] into the house, cut down the fence, entered and arrested people unlawfully. That’s why I’m also worrying.”
An office of the US embassy in Yangon warned US nationals to stay indoors during curfew hours.
On Saturday, the military said arrest warrants had been issued for seven prominent opposition campaigners and warned the public not to harbour opposition activists fleeing arrest.
Video footage showed people reacting with defiance, banging pots and pans to warn their neighbour’s of night-time raids by the security forces.
The military on Saturday also suspended laws requiring court orders for detaining people longer than 24 hours and for searching private property.
What is the worlds’ reaction?
Western embassies urged the military to show restraint.
UN Secretary-General Antonio Guterres said he was “deeply concerned” about reports of the use of excessive force by security personnel and “the reported deployment of additional armored vehicles to major cities”.
Mr Guterres urged Myanmar’s military leaders to “ensure the right of peaceful assembly is fully respected and demonstrators are not subjected to reprisals”, according to a statement released by his office on Sunday.
Tom Andrews, the UN special rapporteur on Myanmar, accused the military of “declaring war” on the people and said the generals were showing “signs of desperation” and would be held accountable.
A statement signed by the EU, the US and the UK said: “We call on security forces to refrain from violence against demonstrators, who are protesting the overthrow of their legitimate government.”