Green MP Caroline Lucas criticised "oppressive policing" for undermining legitimate protest after being cleared of offences linked to an anti-fracking protest.
The Brighton Pavilion MP took a swipe at police for trying to "silence dissent" as she said protest was "the lifeblood of democracy".
Ms Lucas, 53, spoke after she and four co-defendants were cleared of obstructing a public highway and breaching an order under Section 14 of the Public Order Act.
Outside Brighton Magistrates' Court, Ms Lucas told a crowd of supporters and reporters: "Protest is the lifeblood of democracy.
"We are deeply concerned that the right to protest is being eroded and undermined, with legitimate protest criminalised by oppressive policing in an attempt to silence dissent."
Ms Lucas - who was elected as the Green Party's first MP in 2010 - said they were pleased with the acquittals, but that it was "not a victory or cause for celebration".
"We will continue to campaign to end fracking and only celebrate when our world is on the path to a clean energy future," Ms Lucas went on.
Ms Lucas's co-defendants were Josef Dobraszczyk, 22, from Bristol; Ruth Jarman, 50, from Hook, Hampshire; Sheila Menon, 42, from north east London; and Ruth Potts, 39, from Totnes, Devon.
All five faced a six-day trial after being arrested outside energy company Cuadrilla's exploratory oil drilling site in Balcombe, West Sussex, last August 19.
Ms Lucas was among the group of protesters who had linked arms outside the site, which had become a national focal point for anti-fracking protests.
She told the trial she "wanted to express solidarity" by protesting peacefully in a bid to prevent the UK being locked into using more fossil fuels.
The notice under Section 14 of the Public Order Act was imposed, requiring demonstrators to use a designated protest area away from Cuadrilla's entrance.
But in his concluding remarks, district judge Tim Pattinson said the prosecution had failed to satisfy him that Ms Lucas had "the requisite knowledge" about the order being in place.
Although she had been handed the notice, she "found it to be incomprehensible" and later lost it as a banner was unfurled, Mr Pattinson said.
She was also distracted by the arrest nearby of her son Theo "and the obvious pain being caused to him during his arrest", the district judge added.
He went on: "She had no idea where the designated area was.
"My finding on these facts was that the prosecution have failed to satisfy me that Miss Lucas did have the requisite knowledge. I also find, on these facts, that there is insufficient proof of failure to comply."
Mr Pattinson said the notices were handed out in a "somewhat casual manner" and no evidence was called from any of the liaison officers who purportedly served them.
The district judge said the conditions were "lacking in clarity" and that an accompanying map was "inadequate".
He went on: "If criminal prosecutions for failure to comply are envisaged, as they must have been in this case, it is imperative that conditions are clear. These conditions are not clear.
"In short, I have concerns about the notice on the particular facts of this case that I find it to be invalid."
On the obstruction charge, Mr Pattinson said he did not hear any evidence that an "actual obstruction" of a vehicle or person was caused by the protest.
He said: "There was reference to the possibility of obstruction to emergency vehicles but no evidence of any actual obstruction.
"All the defendants said that they would have moved if it had been necessary to permit access to emergency vehicles. None of these defendants was 'locked on' to each other or to any object."
He added: "The obstruction was temporary in nature. No vehicle was obstructed. The road to the north was closed in any event and the defendants were in the driveway as opposed to the main carriageway.
"Therefore on these facts, I am not satisfied to the criminal standard that any of the defendants is guilty of this offence and I formally find each one not guilty."
Cheers and applause broke out in the packed courtroom after the not guilty verdicts were returned.
Following the case, environmental groups hailed the acquittals as a "victory for the right to peaceful protest".
Greenpeace UK executive director John Sauven said: "This verdict is a victory for the right to peaceful protest but more importantly for Britain's growing movement of ordinary people opposing fracking.
"Caroline Lucas had the guts to stand up for what she believes in. In the age of expense scandals and 'cab for hire' politicians, an MP ready to put her neck on the line is a refreshing sight."
Sussex Police, which oversaw the policing operation at Balcombe which cost nearly £4 million, said it respected the verdicts.
Superintendent Lawrence Hobbs said: "The operation was a difficult balancing act throughout and we have been variously reported as 'caving in' to protesters and accused of 'overkill' in the number of officers deployed.
"Right from the outset, we made it absolutely clear that our priority in this operation was safety - for the general public, local residents, protesters, Cuadrilla employees and for own officers and staff.
"We worked with all sides to enable them all to meet their peaceful and lawful objectives, whether they were day-to-day commercial activities or protest."