Mama Qadeer Baloch and his supporters have set up a camp outside the National Press Club here after walking from Quetta to Islamabad over three months.
"I am here on the invitation of the United Nations. I will go to the UN office on Monday and I am sure our demands will be met within the next 15 to 30 days," he told reporters.
He claimed a "total of 19,200 have gone missing from Balochistan". Rights groups say hundreds of people have been detained by security agencies in Balochistan.
Though Baloch was followed by a small band of about 30 supporters during the march, many more joined him in Rawalpindi but most had their faces covered.
More From This Section
Carrying banners and shouting slogans, masked youngsters made a human chain around him as he walked into Islamabad.
"Mahatma Gandhi got his country after a march of 300 km. I am hopeful I will get justice after having walked 3,000 km," Baloch, who is leading the march under the banner of Voice of Missing Baloch Persons (VMBP), told PTI last evening.
The term "missing persons" is used in Pakistan to refer to persons detained without charge by security and intelligence agencies.
The first phase of the protest, a 730-km walk, ended in Karachi in November and the second phase began in mid-December.
Baloch's march has often been compared to the Dandi march of Gandhi. In 1930, Gandhi had undertaken a 390-km march to protest the British salt tax.