Thursday's session before the FIA's International Tribunal in Paris was called after Mercedes appeared to contravene the world governing body's ban on in-season testing by their current car and regular drivers Lewis Hamilton and Nico Rosberg in the 1,000-kilometre test held between May 15 and 17.
Pirelli was summoned on the grounds that it had allegedly failed to inform all the other teams of the test.
In the hearing's opening address on Thursday, the FIA claimed Mercedes were in breach of Article 22 of its sporting regulations, which outlines the ban on in-season testing. In doing so, the FIA claimed, they also breached Article 151c which prevents competitors committing "any fraudulent conduct or any act prejudicial to the interests of any competition or to the interests of motorsport generally".
Paul Harris QC, representing Mercedes, rejected the FIA's stance by insisting it was "irrefutable" that Pirelli controlled the running of the car and the driver for the duration of the test.
He said: "Pirelli did it all. They were in charge of it all and it's obvious why - it was a Pirelli test."
Article 22 defines testing as "any track running time not part of an event undertaken by a competitor entered in the championship".
Harris added: "This was not a test undertaken by Mercedes. They are critical words in the text of Article 22 - 'undertaken by'."
Mercedes' defence also rests significantly on their assertion that they were given permission by FIA race director Charlie Whiting to use their 2013 car - the W04 - at the test.
However, Mark Howard QC, speaking first on behalf of the FIA as the body's legal representative, said Whiting's consent was "irrelevant".
Howard said that Whiting was not authorised to amend Article 22 of the sporting regulations, saying such a move could only be undertaken by the FIA's World Motor Sport Council.
Discussing the sporting regulations more generally, Howard said: "They are not capable of being waived by the FIA's race director, by an opinion that he may give.
"They are regulations that are finally decided on by the World Motor Sport Council - that is the only body which is authorised to decide on the regulations and any amendment to them. They can't be amended during the season.
"So any testing which doesn't comply with regulation 22 can only lawfully take place if a formal dispensation is decided by the World Motor Sport Council."
In their pre-hearing submission, Mercedes revealed they had spoken to Whiting in early May to discuss the prospect of using their 2013 car - as they did not have an older car at their disposal.
Howard said the question asked of Whiting in an email from Mercedes sporting director Ron Meadows on May 2 was "non-specific" in nature.
Howard said Whiting replied to the effect that "such a test would comply with Article 22 provided its purpose was to test Pirelli tyres, but that he would check with the FIA."
Howard added that the communication "wasn't an agreement".
Harris, however, countered that Mercedes had demonstrated an "abundance of caution" by checking the legality of running their 2013 car with Whiting before doing so.
Mercedes further insist that they gained no benefit from the Pirelli test as the data was for the sole consumption of the tyre manufacturer.
Yet Howard claimed telemetry data was transmitted back to Mercedes engineers and that they could not help but benefit.
"Running the car for three days at least informed Mercedes that they did not need to change the cars," he said. "That in itself would be valuable information.
"It is not beyond the bounds of possibility that some defect would have become apparent in the three days' track testing, and it cannot seriously be suggested that Mercedes would not seek to remedy that defect.
"We would suggest that it is difficult to say Mercedes obtained no benefit from the test."
And as part of his summing-up, Howard said: "By testing the 2013 car for for three days with current drivers, Mercedes may be said to have gained an unfair advantage."
In response, Harris said Mercedes' access to telemetry was purely to ensure the safety of the car and driver while on track, and cited similar tests conducted by Pirelli with Lotus and Ferrari in recent years, in which the F1 teams also had access to car data.
Ferrari were initially asked to respond to an FIA request for information relating to today's hearing over the tyre test they conducted with Pirelli earlier this year - held between the grands prix in Bahrain and Spain - but as they used a 2011 car and test driver Pedro de la Rosa they were not summoned to appear before the tribunal.
However, Harris hinted at dissatisfaction at that decision by saying he did "not accept that [by] Ferrari running a 2011 car, that that car does not confirm substantially to the 2012 or 2013 regs".
He added: "The changes between 2011 and 2013 cars are minuscule."
Mercedes team principal Ross Brawn later gave evidence, in which he repeatedly called for "perspective" over the significance of the data gathered by his team over the course of the three days.
Brawn, who has previously revealed that the final decision to agree to the Pirelli test was his alone, said access to telemetry data was an "inevitable consequence" of the work being done, but said Mercedes sought to keep their exposure to it "at a minimum".
The Englishman conceded that undergoing a 1,000km test in a 2013 car "could be perceived as an advantage", but said that nothing was learned that had not already been established during the weekend of the Spanish Grand Prix the preceding weekend.
"The level of competence of top Formula One teams is such that there is no correlation between mileage and reliability," said Brawn. "There was no advancement in our knowledge post the Pirelli test than before the Pirelli test.
"The majority of things the engineers were looking for was a malfunction. The car is fairly mature at the time we've done this."
He added: "We didn't know what the tyres were and we didn't know what the detailed objectives were of what Pirelli were doing.
"We always work quite frankly on the principle that no information is better than bad information.
"So I don't see how we could have used any data that resulted from the Pirelli test."
Brawn was the most high-profile witness for the Mercedes delegation, while Pirelli motorsport director Paul Hembery present for the Italian tyre manufacturer, which was due to submit its statement to the hearing later this afternoon.
Legal representatives of Ferrari, McLaren and Williams were in attendance at the hearing presided over Edwin Glasagow QC, as was Red Bull team principal Christian Horner, who has been one of the fiercest critics of last month's test.
Red Bull and Ferrari lodged a protest after they learned of the tyre test during the weekend of the Monaco Grand Prix late last month - almost a fortnight after the test had been conducted.
In his statement before the hearing on Thursday, Pirelli's lawyer Dominique Dumas repeatedly asserted that the FIA had no jurisdiction over the tyre supplier.
"Pirelli is only exercising the rights which have been conferred upon it by the FIA," Dumas said, referring to a provision in its contract which permits it to request 1,000km test sessions with current teams.
"We are unable to understand the disciplinary process. Ferrari and Red Bull Racing have confirmed that they have no grievances against Pirelli.
"The claims are unfounded because it has been recognised that Pirelli has not violated the international sporting code.
"Pirelli has no FIA licence. It is a supplier amongst many other suppliers."
While not subject to the disciplinary processes surrounding Article 22, FIA counsel Howard said Pirelli was still bound by the terms of Article 151c covering acts "prejudicial" to the sport.
Pirelli - along with Mercedes - has also been criticised for failing to disclose details of the Barcelona test to other the other F1 teams.
Howard earlier said there was "no reason" for Pirelli to not communicate its plans to other teams, and "no reason" for Mercedes to not check that this had been done.