Voting on the amendment is likely to take place on Monday, and it would require the support of two-thirds of the MPs present for it to be passed.

Earlier in the day, Prime Minister Manmohan Singh is reported to have appealed to SP chief Mulayam Singh Yadav to break the logjam. While Mulayam is learnt to have expressed his party’s opposition, the SP offered an exit route to the government by staging a walkout in the Rajya Sabha to facilitate discussions on the issue.

The SP is likely to participate in the discussions on Monday to register its protests against the Bill, said sources. The party also asserted that it would continue to support the UPA government.

“There is no relation between today’s events in Rajya Sabha and continuance of our support. Our support continues to the government as of now,” said Ram Gopal Yadav, SP’s leader in the Rajya Sabha, after leading the walkout.

“Today’s day is a black day in the history of parliamentary democracy for the simple reason that a patently unconstitutional amendment is being considered...” he added.

Trouble began on Thursday immediately after Minister of State for Personnel V Narayanasamy introduced the Bill, making a brief statement on why the legislation was required. Seven of the nine SP MPs trooped into the well, shouting slogans against the proposed legislation.

After his repeated requests to the SP members to return to their seats yielded no results, Deputy Chairman P J Kurien directed SP MP Arvind Kumar Singh to leave the House for causing disruptions. Singh, however, refused to leave and continued shouting slogans.

When the House re-assembled after a brief adjournment, a similar situation prevailed, prompting Kurien to issue repeated warnings to the member to leave or face eviction by marshals. As Singh refused to relent, Kurien asked Ram Gopal Yadav to persuade the member to leave.

Yadav then argued that if the chair wanted to expel all the SP members, it should take the “sense of the House” on it. “If there is sense of House, then we have no objection. We will leave the House,” he said.

However, parliamentary affairs minister Kamal Nath said the member who had been named should first withdraw before any other thing could be considered.

SP member Naresh Agrawal suggested that since his party MPs were defying Rule 255, under which the chair can direct any member to withdraw from the House for the rest of the day, Kurien should invoke Rule 256 under which members can be suspended. The deputy chairman expressed surprise at this argument, terming it as a “strange” request for “greater punishment”, and insisted that the named member should leave first.

When the House re-assembled after a 15-minute break, the other SP members continued slogan-shouting. “If this continues, all members should be asked to leave,” said Nath.

Kurien pleaded with the protesting members to resume their seats. “Do you want me to ask you to leave? Don’t make me do this. You are my friends. I will invoke Rule 255 again. I am going to do this,” he said, and then named another member, Choudhry Munabbar Saleem, and asked him to withdraw.

When he threatened to call the marshals to oust the member, BJP and Left MPs opposed the move. Finally, all SP members staged a walkout.

Meanwhile, initiating the discussion in the House, Arun Jaitley, Leader of the Opposition in the Rajya Sabha, while supporting the amendment, said his party wanted changes to the Bill so as to ensure that it does not create reverse discrimination. He also raised the issue of efficiency to be linked to the promotions.

“My party is fully committed to the concept of social justice,” he said, adding that the BJP always supported affirmative action for upliftment of those who have been historically deprived of opportunities.

He urged the government to reconsider the provisions in the Bill concerning Article 335, saying the Bill in its present form runs the risk of being struck down by courts. “As Parliament, we must remember while exercising our power, we cannot change the basic structure of the Constitution. We can only amend it or alter it,” he said.

Among the parties, the BSP, CPI(M), CPI, Trinamool Congress and JD(U) supported the Bill while Shiv Sena did not support it.

BSP chief Mayawati, who has been pushing for the Bill, blamed the Congress for the plight of SCs and STs in the country and advocated reservation for economically-weaker sections of upper castes as well as backwards.