Pinheiro's report said initial indications by Myanmar's military government of a willingness to address human rights abuses had "disappeared".
He added that more than 700 people arrested during last September's anti-government demonstrations, including a number of monks, were reportedly still in prison.
But Pinheiro's report was rejected by Myanmar's ambassador to the UN, saying the military government allows freedom of expression and assembly.
"There are no political prisoners in Myanmar"
U Wunna Maung Lwin, Myanmar ambassador
U Wunna Maung Lwin said the allegations in the report were unobjective and dictate to the Myanmar government on matters falling within its domestic jurisdiction.
"There are no political prisoners in Myanmar. The individuals who are serving prison terms are those breaking the established laws of Myanmar," U Wunna Maung Lwin said.
He added that Myanmar security forces had restored peace and stability in the wake of last year's protests.
Unveiling his findings, Pinheiro said he believed there to be about 1,850 political prisoners behind bars in Myanmar jails as of January.
He said restrictions on freedom of movement, expression, association and assembly continue to be reported in Myanmar.
|Pinheiro said initial signs of any willingness |
to address abuses had "disappeared" [EPA]
He spoke of allegations of arrests and harassment of individuals accused of communicating information to the foreign organisations and media.
Pinheiro was particularly concerned about accounts of political activists, human rights defenders and journalists being searched and detained for reasons including possession of copies of his reports to the UN.
International pressure mounted on Myanmar after its violent crackdown on protesters last year, including from its Asean neighbours.
But calls for political reforms to include the opposition have largely fallen on deaf years and the voices urging change have grown noticeably quieter.
Thai PM visit
On Friday, the new Thai prime minister, Samak Sundaravej, made a "courtesy" visit to the Myanmar capital Naypyidaw for talks with the country's top leader, Senior General Than Shwe.
Wichianchote Sukchotrat, a Thai government spokesman, said Samak's visit was "purely to introduce himself".
"The Thai government will not raise the issue of Myanmar's political development because it's a sensitive issue and Myanmar's government is never willing to talk," he said.
Another senior government official, speaking on condition of anonymity, said Thailand - one of the biggest investors and trading partners in Myanmar - would congratulate the ruling generals on their plan to hold a constitutional referendum in May.
The military says the referendum will clear the way for multi-party elections in 2010 as part of its "road map to democracy" but local and international observers have criticised the plan as a thinly veiled attempt to hold on to power.
The rules of the plan block Aung San Suu Kyi, the detained opposition leader, from running in any elections.
Samak does plan to discuss the flight of political dissidents into the kingdom as well as the hundreds of thousands of refugees and economic migrants living there, the Thai official added.
However, the Thai leader is apparently carrying a message from the US to the generals.
Christopher Hill, the top American official on East Asian affairs, held talks with Samak last month and asked him to deliver an undisclosed message from Washington.