Myanmar's government has said 10 people died and around 3,000 were detained in the pro-democracy protests - the biggest in nearly two decades - which sparked international outcry and calls for reforms in the country.
Call to release inmates
But Amnesty International estimates 700 political prisoners remain in detention, including 91 detained during the recent protests, and are demanding they all be released.
The human rights group has also accused the Myanmar authorities of being behind the disappearance of at least 72 people.
"Until the generals' military hardware is crumbled, they won't listen to anyone"
Oomlwin, Yangon, Myanmar
Send us your views
The authorities must "immediately and unconditionally release all of those who were arrested for exercising their right to freedom or expression or assembly during the crackdown, as well as all prisoners of conscience held before the recent events," the group demanded.
Meanwhile Human Rights Watch, the New York-based organization, said the UN security council should "redouble efforts to prod Burma's [Myanmar's] generals into starting a genuine political dialogue and ending human rights abuses".
Both groups called on Myanmar's military leaders to co-operate with Pinheiro, who had been barred from visiting the country since November 2003, and give him full and free access to political prisoners.
Pinheiro's visit comes days after a mission by Ibrahim Gambari, the UN special envoy, who met Aung San Suu Kyi, the country's detained opposition leader who has spent 12 of the last 18 years under house arrest in Yangon.
Gambari's visit ended with the UN declaring that progress had been made, and Aung San Suu Kyi later met a junta official and members of her political party, the National League for Democracy (NLD).
Nyan Win, the NLD spokesman, said they were hoping for a visit at their Yangon headquarters from Pinheiro.
"We expect to meet him but we have not heard anything from the authorities," he said.
Pinheiro, who leaves Myanmar on Thursday, will travel to Naypyidaw, the isolated new capital, on Monday, a Myanmar government official said.
Analysts and residents appear divided over whether the Friday's meetings represent a step towards real change in Myanmar, or if it was just a move by the military leaders to appease the international community.