Labour MPs who have sought to depose Jeremy Corbyn are guilty of mugging a decent man "in cold blood", Baroness Chakrabarti has told the BBC.
The ex-director of civil rights group Liberty, who Labour recently nominated for a peerage, accused MPs of failing to see it is "time for change".
The outcome of the Labour leadership election between Mr Corbyn and Owen Smith will be announced on Saturday.
The winner will be unveiled at the party's annual conference in Liverpool.
Speaking to BBC's Newsnight, Baroness Chakrabarti hit back at the 172 Labour MPs who helped to trigger the leadership challenge against Mr Corbyn when they supported a no confidence motion following the EU referendum.
The Labour peer said she did "not approve" of a 67-year-old man "being mugged in broad daylight, in cold blood, by people who don't see that it is time for a change".
She added: "I think it's going to be incredibly important that the Labour Party unites behind whoever wins.
"I hope and believe it will be Jeremy Corbyn."
Mr Corbyn is hot favourite to see off the challenge from rival Mr Smith, prompting speculation about what former cabinet ministers who resigned following the Brexit vote will do if he does win.
Lisa Nandy, the former shadow energy and climate change secretary, indicated she would only return to the Labour frontbench if Mr's Corbyn leadership team changed their ways.
She told Newsnight: "I resigned from the shadow cabinet not because of policy differences but because of a fundamental disagreement with a sort of approach that says that this is a battle, this is a war, and it must be won and dissenting voices must be silenced."
A political party or shadow cabinet "simply cannot survive if you refuse to hear dissenting voices and work as a team to try and resolve differences," she added.
"Then it is not a shadow cabinet. It is quite simply just a fan club," she said.
Meanwhile, Lord Hain, the former Labour cabinet minister who knew Mr Corbyn as a "junior" anti-apartheid campaigner in the 1970s, says the party is facing the gravest crisis in its history.
"We are facing the biggest crisis that the party has faced, even compared with the Ramsay MacDonald defection 80 years ago and the crisis that followed the 1979 defeat and the breakaway by SDP and the defection of significant numbers of Labour members and leaders.
"This is more serious."
He added: "The hard left around Jeremy have never had control of the party before, they have never had control of the leadership and they have never had control of the organisation.
"If they get that then they will have achieved what they want to achieve, which is control of the party rather than winning the country."