A special UN Human Rights envoy has met lawyers for Burma's opposition party on the first day of a five-day visit to the country.
Tomas Ojea Quintana has said that with elections due sometime in 2010, Burma is facing a critical year.
Opposition groups welcomed the UN visit, with one activist saying human rights in Burma were "at the abyss".
It comes two days after pro-democracy leader Tin Oo was released following seven years in prison.
A lawyer for the National League for Democracy party (NLD) told the BBC Mr Quintana had spoken with them in Rangoon for about an hour.
The lawyer said the group had discussed Burma's legal system and the detention of their party leader, Aung San Suu Kyi.
Senior NLD member Win Tin called on Mr Quintana to "be decisive and perform his duties in the strictest manner without falling prey to the lies of the government".
"Myanmar's human rights conditions are at the abyss. The government continues to violate human rights and they don't have the will to alleviate human rights conditions," the Associated Press news agency quoted him as saying.
Mr Quintana is also expected to meet several ministers in the military government and to visit Rangoon's notorious Insein prison.
But he has not been granted an audience with the man who makes all key decisions in Burma - General Than Shwe.
Mr Quintana has also asked for permission to visit Ms Suu Kyi in detention, but on Monday said that he had not yet received a response to his request.
Tin Oo, vice-president of the NLD was released from prison on Saturday, having been in prison since 2003.
As he visited NLD headquarters on Monday, he said he was optimistic that "things can be resolved" through Mr Quintana's visit.
He said he thought Ms Suu Kyi could be freed soon as she had shown good behaviour under house arrest, a condition of her release.
On Friday, Gen Than Shwe said elections - the first in two decades - would be held "soon".
But Tin Oo has not yet said whether the party will take part.
There are some 1,200 such political prisoners in Burma and their release will be one of the main items on Mr Quintana's agenda, says the BBC's South East Asia correspondent Rachel Harvey.
It is perhaps no coincidence that a high profile detainee such as Tin Oo won his freedom just ahead of the UN envoy's arrival, says our correspondent.
But the Burmese leadership is adept at giving hints of progress on reform when international attention is focused in its direction, she adds.
Tin Oo was at the end of his latest period of detention, so his release does not represent a major concession on the part of the government.