GENEVA — Myanmar is reversing democratic reforms at an accelerating rate in an election year that will define its future, a United Nations investigator monitoring developments in the country said on Wednesday.

“There are worrying signs of backtracking, and in some areas, backtracking has gained momentum,” Yanghee Lee, the special rapporteur on Myanmar, told reporters in Geneva, drawing attention to harsh treatment and arrests of people who criticize the government or the country’s politically dominant armed forces.

Myanmar officially acknowledges 27 political prisoners, but 78 other activists have been detained on political charges, and 200 more are on trial, Ms. Lee said. Human rights activists face regular surveillance, and some have been imprisoned under outdated defamation and national security laws, with a “chilling effect” on activism, she added.

President Thein Sein’s government, which won praise for opening up Myanmar to reform, is preparing for elections this year and is promising a referendum on amendments to the Constitution, but government leaders appear to be stonewalling calls to make the Constitution more democratic, Ms. Lee said.