Nick Clegg has insisted the "highest possible safeguards" will be built into any new surveillance powers introduced by the government but said new technology meant the debate was needed.
Speaking on the BBC Radio 4 World at One programme, the deputy prime minister refused to elaborate on what the safeguards might be, insisting it was appropriate for the plans to be published first.
But the Liberal Democrat leader said what was being discussed was a case of updating existing powers to take account of new technology - such as internet telephone service Skype - which he said criminals and terrorists could use to subvert surveillance laws already on the books.
News of the proposals has sparked fierce opposition from civil liberties campaigners, including Tory MP David Davis.
Clegg told the programme: "I think it is very important people hold off making their judgement until they see the proposals.
"There has been a lot of speculation, some of it inaccurate, over the last couple of days. I happen to think it is right to have a debate about what we do as a society as criminals exploit new technologies.
"People should be reassured we are not going to ram something through Parliament. All along we will be guided by some very simple principles."
Clegg said this will mean ensuring there is no new Government database and consulting widely - not just on new powers but on whether existing powers are already sufficient.
The deputy prime minister said the police would not be given new powers to look at people's emails but said powers may need to be updated to "keep pace with the use of new technology".
He added: "Anything in this area is highly sensitive and it is good we are having this kind of wider debate."
Mr Clegg highlighted existing police powers which can involve the release of phone records detailing who has made a call, where and when that call was made - but that 'voice over internet protocol (VOIP)' calls, such as those made using Skype, were not recorded.
The Lib Dem leader - who campaigned in the last election against a new government surveillance database - said: "We have to confront as a government it is now possible to communicate with each other using different routes and we do need to update the means and powers that already exist on the statute books to reflect that change in technology.
"There will be the highest possible safeguards. What I can't do is tell you what those are in detail because the proposals have not been published yet.
"I'm absolutely clear... we will not return to the bad old days under the Labour Party.
"This will be an open, consultative and properly scrutinised process."
But Labour leader Ed Miliband slammed the government for the way the issue had been handled, saying they had "failed" to approach the issue sensitively.
On a local election campaign stop in Leeds, he said: "Once again we see a very sensitive issue being spectacularly mishandled by this government.
"It is unclear what they are proposing. It is unclear what it means for people. It is always going to lead to fears about general browsing of people's emails unless they are clear about their proposals, clear about what they would mean, clear about how they are changing the law.
"And I say to the prime minister: he has got to get a grip on this government. He has got to get a grip on the way his government operates and the way that policy is made.
"If we are talking about stopping people committing murders and terrorist acts, there have always been powers in place and we will look at any proposal the government has.
"When you are dealing with sensitive privacy issues, with people's fears about what a government database might mean or what the government will be looking at you must be incredibly sensitive about the way you handle the proposals, the way you set them out and clarity about what they mean.
"The government has failed that test."
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