The House of Lords is voting on controversial moves to extend the terror detention limit to 42 days.
The House of Lords has rejected the government's controversial plans to extend the period terrorist suspects can be held without charge to 42 days.
It is widely predicted that peers will defeat the government proposal, which was backed by MPs in June.
Peers threw out the plans, which got through the Commons by only nine votes in June, by 309 votes to 118.
Crossbencher Lord Dear, who has tabled an amendment to keep the pre-charge detention limit at 28 days, called ministers' plan "shabby" and "flawed".
Last week ministers said the plan would return to MPs if defeated but there has been speculation it might be dropped rather than risk a Commons defeat.
But Lib Dem Lord Carlile, the official reviewer of terror laws, said it was an "appropriate" attempt to save lives.
The government failed in 2005 to extend pre-charge detentions to 90 days.
The House of Commons backed the extension in terror detentions by just nine votes in June, with 36 Labour MPs rebelling against the government.
'Unworthy'
The Conservative and Liberal Democrat leaderships both oppose the plan.
Supporting Lord Dear's amendment are the Conservative frontbencher Baroness Neville-Jones, a former Joint Intelligence Committee chairwoman, Lib Dem frontbencher Lord Thomas and Labour QC Baroness Mallalieu.
Opening the debate, Lord Dear said: "This attempt to appear tough on terrorism, I believe, is a shabby charade which is unworthy of a democratic process and we should reject it."
He said there was "no proven case" for changing the limit, that the legislation was "fatally flawed, ill thought-through and unnecessary" and would "further erode fundamental and legal rights that have been the pride of this country for centuries".
I see no thin end of the wedge argument here Lord CarlileTerrorism law reviewer Q&A: Terror plans
Baroness Neville Jones, the shadow security minister, warned peers the legislation could hinder efforts to work with Muslim communities and said the number of safeguards introduced "not only make it unworkable, they also make it constitutionally worrying".
She said letting Parliament vote on whether to allow detention beyond 28 days was asking it to act in a "quasi-judicial manner".
For the Lib Dems, Lord Thomas said such a vote, based on a statement from the home secretary, could prejudice a suspect's trial adding: "That is not only unconstitutional; it is contemptible."
However, Lord Carlile told peers he would not take part in the vote as he did not want to take advantage of his position, but supported the 42-day limit.
'Heavy heart'
He said he dreaded a major terrorist attack, such as a plane or bomb being driven into a crowd of thousands, and did not want to have to make changes to the law in the aftermath.
"I see no thin end of the wedge argument here. What I see is finality in the law of detention with appropriate detention," he said.
He said he believed the extension would effect adversely a "maximum maybe of five or six people in the next four or five years".
HAVE YOUR SAY It is necessary to protect the country from terrorism but not at the cost of human rights Helen Toalster, Manchester Send us your comments
"This is not the end of civil liberties as we know it. And it could have a consequence of saving many lives at home and abroad, including people in those humanitarian camps with the UN and others keep abroad."
Former lord chancellor Lord Falconer has said he will vote against the government "with a heavy heart", while the former Conservative chairman Lord Tebbit has said he will vote with the government.
Lord Tebbit warned his own party they may "rue this day" if they won the vote, then won the next general election and found they needed the powers.
90 days
David Davis, the Tory MP who resigned in protest at the Commons vote to fight for his seat on a civil liberties platform, is watching the debate from the steps of the throne.
Peers began debating the Counter-Terrorism Bill's detailed committee stage last week.
Last week, the government insisted the 42-days plan would return to the Commons, with Commons Leader Harriet Harman saying it would be dealt with in the "usual way".
If peers reject the plan, ministers can use the Parliament Act to force it into legislation.
But BBC political editor Nick Robinson reported last week that the government had decided against this, as it would be time-consuming and controversial.
The government failed in 2006 to extend pre-charge terror detentions to a maximum of 90 days. Instead, the current 28-day limit was agreed as a compromise.
* BBC Parliament is broadcasting the House of Lords live . You can watch it on Freeview, cable and satellite television. You can watch proceedings in both the Commons and Lords live on the BBC website.
