Pro-hunt campaigners have launched a legal challenge to the ban on hunting with hounds.
About 1,000 pro-hunting protesters demonstrated on Thursday
Members of the Countryside Alliance have lodged papers with the High Court in London seeking a judicial review.
Lawyers claim the 1949 Parliament Act, used to force through the ban, is itself invalid because it was never approved by the House of Lords.
The Countryside Alliance's Simon Hart said the case could have far-reaching "constitutional implications".
He pledged to fight what he called the government's "abuse of power" and the "political thuggery" of MPs who pushed for a ban.
Mr Hart said the battle had now moved "out of the snake pit of the House of Commons" and into "the more intelligent" arena of the courts "where evidence is listened to, not ignored as it was in Parliament".
The alliance's application will be studied by a judge who will decide whether there is an "arguable case" which should go to a full High Court hearing.
Tony Blair said the ban would now be an issue at the general election, expected early next year.
"I am afraid the views on both sides are very, very entrenched," Mr Blair said.
"We will now go to the courts and I have no doubt it will be an [election] issue, but it always was going to be an issue."
USE OF THE 1949 PARLIAMENT ACT
1991: War Crimes Act
1999: European Parliamentary Elections Act
2000: Sexual Offences Amendment Act
2004 Hunting Bill
Lord Donaldson, former Master of the Rolls, said the alliance had a powerful case.
"My personal view is that it is right, 100% strength. Whether the courts will agree is a matter for them, and we will just have to see," he told BBC Radio 4's Today programme.
The BBC's Home Affairs correspondent Andy Tighe described the alliance's legal move as a "very big constitutional challenge", which, if it succeeded, could throw previous occasions when the 1949 Parliament Act had been used into doubt.
That could potentially lead to the reversal of laws on the gay age of consent, the election of MEPs and the prosecution of war criminals, he said.
The Countryside Alliance also claims the ban infringes European human rights legislation and is planning a separate legal action once the Hunting Bill comes into force on 18 February.
It says compensation should be paid to those people who could potentially lose out as a result of the ban, such as farriers, saddlers and feed merchants.
Campaigners are also threatening civil disobedience and say they will attempt to unseat Labour MPs who voted for the ban at the general election.
It has been reported that landowners are preparing to stop the Ministry of Defence using their land for training, block the siting of electricity pylons
and stop water companies using their land to spread sewage.
Rural Affairs Minister Alun Michael urged campaigners not to "punish society" with "irresponsible, illegal and completely out of proportion" acts of civil disobedience.
Mr Michael also said he was confident of seeing off the legal challenge.
Pro-hunt Labour peer Baroness Mallalieu said the parliamentary system had failed to protect a minority from "unfair legislation", so they would look to the courts to do so.
The group has also pledged to test loopholes in the ban when it comes into force in February.
And it says 50,000 people have said they are prepared to break the ban and continue hunting, "in the full knowledge they will be arrested".
The application for a judicial review is being lodged by Countryside Alliance chairman John Jackson, farrier's wife Mair Hughes, from Rhondda, south Wales, and Bicester Hunt member Patrick Martin.