Undercover police are running a network of hundreds of informants inside protest organisations who secretly feed them intelligence in return for cash-in-hand payments, according to evidence handed to the Guardian.

In the material, the police claim to have infiltrated a number of environmental groups and say they are receiving information about leaders, tactics and detailed plans of future demonstrations.

The dramatic disclosures are revealed in almost three hours of secretly recorded discussions between covert officers, claiming to be from Strathclyde police, and Matilda Gifford, an activist from the protest group Plane Stupid. The officers attempted to recruit Gifford as a paid spy after she was released on bail after a protest at Aberdeen airport last month.

Gifford, 24, said she recorded the meetings in a bid to expose how police seek to disrupt the legitimate activities of climate change activists. She met twice with the officers, who said they were a detective constable and his assistant. During the taped discussions, the officers:

• Indicate that she could receive tens of thousands of pounds to pay off her student loans in return for information about individuals within Plane Stupid.

• Say they will not pay money direct into her bank account because that would create an audit trail that would leave her compromised. They say the money would be tax-free and add: "UK PLC can afford more than twenty quid."

• Accept that she is a legitimate protester, but warn that her activity could mean she will struggle to find employment in the future and result in her receiving a criminal record.

• Claim they have hundreds of informants feeding them information from protest organisations and "big groupings" from across the political spectrum.

• Explain that spying for police could assist her if in the future she were arrested. "People would sell their soul to the devil," an officer says.

• Warn her that she could be jailed alongside "hard, evil" people if she received a custodial sentence.

The meetings took place in a Glasgow police station last month and in a supermarket cafe on Tuesday this week. Gifford used a mobile telephone and device sewn into her waistcoat to record the officers when they offered what they described as a "business proposal" that she should think of as a job.

They intimated that in return for regular updates on Plane Stupid's plans she could receive considerable sums in cash.

When lawyers acting for Plane Stupid contacted Strathclyde police this week to establish the identities of the detective constable, they were initially told by the human resources department there was no record of his name.

However, when the Guardian contacted the force, they acknowledged that officers had met with Plane Stupid activists.

In a statement last night, Assistant Chief Constable George Hamilton said the force had "a responsibility to gather intelligence" and such operations were conducted according to the Regulation of Investigatory Powers Act (Ripa). The force would not comment on the identities of the officers.

"Officers from Strathclyde police have been in contact with a number of protesters who were involved with the Plane Stupid protests including Aberdeen airport," he said. "The purpose of this contact has been to ensure that any future protest activity is carried out within the law and in a manner which respects the rights of all concerned."

It is known that at national level, a confidential intelligence unit has been set up by the Association of Chief Police Officers (Acpo) with the power to operate across the UK, mount surveillance and run informers, with the intention of building up a detailed picture of "domestic extremism".

Gifford's lawyer, Patrick Campbell, said: "I have very considerable concerns about these events. There appears to be a covert operation that is running in some way with, or using Strathclyde police's name. There appears to be a concerted effort to turn protesters to informants and possibly infiltrate peaceful protest movements.

"The methods employed are disturbing, and more worrying yet is the lack of any clearly identifiable body responsible for this. These individuals seem to have some kind of police support or at the very least connections with the police – the access to police stations confirms that – but my concern is the lack of accountability and the threat to the individual and her right to protest."

Gifford intended to meet the officers for a third time yesterday, taking a lawyer with her. But the officers did not appear at the rendezvous. However, she said that later in the day she was approached by the detective constable, who said he was disappointed in her. The man got into a waiting car that drove off, leaving Gifford feeling shaken and intimidated.

Gifford said today that the initial approach from the officers was "an opportunity that fell out of the sky". "Recording them seemed like the obvious thing to do. I was keen to find out what they had to offer, what they wanted to find out, and feed that back to the group in case other members of Plane Stupid were approached."

But she added: "In a sense it rippled out to something much wider, in terms of not knowing who the two men are. It's opened up in all these murky directions."