Freedom of expression has been worsening since the Nobel peace laureate's administration took office in 2016, with prosecutions creating a "climate of fear" among journalists, Human Rights Watch (HRW) said on Friday.
"Aung San Suu Kyi and the National League for Democracy promised a new Myanmar but the government still prosecutes peaceful speech and protests and has failed to revise old oppressive laws," Linda Lakhdhir, Asia legal adviser at HRW and the report's author, said in a statement.
There was no immediate comment from the Myanmar government.
Under nearly 50 years of notorious military rule, the country's government had placed severe restrictions on freedom of expression.
Reforms undertaken by the quasi-civilian administration that came to power in 2010, including the abolition of censorship, had "positive implications for speech and assembly", HRW said in its report, titled Dashed Hopes: The Criminalisation of Peaceful Expression in Myanmar.
However, the government led by de facto civilian leader Aung San Suu Kyi had made "only marginal changes" to oppressive legislation and continued to use "overly broad, vague, and abusive laws" to prosecute peaceful speech and assembly, it said.
"Aung San Suu Kyi's government has had a real opportunity to abolish the tools of oppression used by the military juntas, but has instead used them against peaceful critics and protesters," Lakhdhir said, urging the government to reverse course and take steps to protect freedom of speech and assembly.
Speaking to Al Jazeera from Yangon in southern Myanmar, Lakhdhir focused on the plight of journalists in Myanmar, specifically when they cover issues considered sensitive by the government.
"There are certain topics that seems to be off limits such as criticising the military, talking about military abuses, criticising the government or government officials and talking bout corruption," said Lakhdhir.
"If you report on ethnic armed groups or travel to conflict areas to report then you risk getting arrested."
Myanmar free speech group Athan, whose report was quoted by HRW, said some 140 cases had been filed since 2016 under the Telecommunications Act, at least half of which involved prosecution for peaceful speech.
Parliament made some amendments to section 66(d) of the act, which punishes anyone who "defames" someone using a telecommunications network with up to two years in prison, but rejected calls to repeal the provision.
Reporters were especially vulnerable to prosecution and attacks, HRW said, with threats coming from authorities as well as nationalists and "militant" supporters of the government or army.
"The result has been a climate of fear among local journalists," the HRW report said.
Laws criminalising defamation, the Official Secrets Act, the Unlawful Associations Act, the 1934 Aircraft Act, and section 131 of the Myanmar Penal Code have all been used against journalists in recent years.
Reuters news agency reporters Wa Lone and Kyaw Soe Oo were sentenced to seven years in prison in September 2018 under the colonial-era Official Secrets Act. They had been working on an investigation into the massacre of 10 Rohingya Muslim men and boys in Rakhine state.
The duo's sentence provoked international outcry over media freedom in Myanmar with calls echoing across the world for their immediate release.
On Friday, weeks after Yangon's High Court rejected an appeal by the journalists, their lawyers filed an appeal with the country's highest court to overturn the convictions - the last chance of a reprieve through the legal system.
Under Myanmar law, Supreme Court judges hear appeals individually, giving lawyers for the journalists several options.
The process is expected to take several months. The only other chance of early release comes from a presidential pardon.
Rights groups have heaped pressure on Aung San Suu Kyi to use her leverage to secure a pardon.
But she has refused to intervene in the case in favour of the reporters, describing it as an issue for the courts.