Around 20-25 MPs, including National Scheduled Caste Commission chief and Congress MP P L Punia and LJP president Ramvilas Paswan, sat on a dharna, first at the main entrance of the Parliament building and later in front of Mahatma Gandhi's statue.
Carrying placards, they raised slogans in support for their demand.
Later Paswan told reporters the SC/ST Parliamentary Forum had been assured that the constitutional amendment bill in this regard would be cleared in this session itself.
"If this is not allowed to happen, we will agitate outside also to get our demands accepted," he said.
During the all-party meeting convened by Prime Minister Manmohan Singh earlier this month, it was decided that wider consultations would be held on the issue to forge a consensus before the bill is tabled in Parliament.
While the BSP spearheaded demand for early introduction of the Bill, its arch rival Samajwadi Party strongly objected to the very concept of reservation in promotion.