The campaign includes academics, science editors and broadcasters
Libel laws in England and Wales are being used to bully people into silence and quash dissent, campaigners including comic Dara O'Briain claim.
O'Briain told the Law Society he backed calls to reform the "ridiculous system" that was attracting "libel tourists".
The Mock the Week host was speaking as academics, medical and science editors and human rights activists prepared to lobby MPs and peers to reform the laws.
Justice Secretary Jack Straw is working on proposals for reform.
Last month he said it was important that people were able to take action if they were seriously defamed, but typically huge lawyer's fees and payouts made the current system "unbalanced".
He said journalists and academics were finding their freedom to offer fair comment was being restricted by big corporations.
Burden of proof
So-called libel tourists from abroad are using England and Wales's libel law to try to sue critics, if they can show someone in England or Wales has bought, read or downloaded potentially defamatory material about them.
Campaigners say the laws make it easier to win than in jurisdictions such as the US, which put greater emphasis on freedom of speech.
Critics say it is wrong that in libel cases the heavier burden of proof lies with the defendant.
LIBEL LAW - ENGLAND & WALES
Civil law on defamation covers both libel (published) and slander (spoken)
To be defamed means the statement exposed claimant to hatred, ridicule or contempt; caused them to be shunned; lowered them in the minds of "right-minded" citizens; disparaged them in their business, office, trade or profession
Claimant has to prove material is defamatory and refers to them, but not that it was false. Burden is on defendant to offer a defence
Defendant has to prove either the material was true; was fair comment made honestly in the public interest; was reported correctly from Parliament or court and is immune from liability; was reported, without malice, from other protected arenas such as a press conference; that amends have been made and accepted
Claimants have to show material published is defamatory and that it is "reasonably understood" to refer to them. But they do not have to prove the material is false.
It is up to the defendant to prove the statement is true, or build a case around a series of other defences in the law.
Speaking at the campaign launch O'Briain said: "The libel laws which were initially set up to protect the reputation of individuals at a time when companies weren't the entities they are now are being used by companies to essentially quash dissent and to destroy criticism.
"That's a major problem. Companies can basically bully people out of saying bad things about their products and services."
He added the "ridiculous" system meant discussion of issues in the public interest, such as medical practices or the benefits of a new pill, were being prevented by the fear of libel.
New Scientist editor Roger Highfield agreed the libel laws were having a "chilling effect on the discussion of medical therapies".
And author AC Grayling said it was a "profound injustice" that big organisations could "buy the silence of other people".
Broadcaster Nick Ross added the laws amounted to "bullying done under the veneer of respectability and decency".
"The law is simply on the side of the devil, as far as I can see," he said.
Tracey Brown, managing director of Sense About Science, one of the organisations calling for reform, said the fear of libel was "so damaging".
Human rights groups were holding back on criticising companies, critical biographies were being hindered, and US newspapers were considering blocking users with UK ISP addresses, she said.
Ms Brown added there was "international embarrassment" over the laws, which were also causing medical journals to hold back publication of reports, and blocking critical reviews of misleading therapies.
The Libel Reform Campaign - backed by English PEN, the Index on Censorship, Sense About Science and Reporters Without Borders - said the cost of a libel trial was often in excess of £1 million - some 140 times more expensive than cases elsewhere in Europe.
Speaking before the launch Stephen Fry said the current situation with "archaic, unfair and illiberal libel laws" was making England a "global laughing stock".
Freedom of speech campaigners have suggested that libel payouts could be capped at £10,000, and apologies made the main remedy.
There have also been calls for the burden of proof to be shifted, so claimants have to demonstrate damage.