The MEPs said they wanted to show "full solidarity" with the two whistleblowers, Antoine Deltour and Raphaël Halet, and journalist Edouard Perrin.
Perrin was acquitted of all charges back in June, while both Deltour and Halet were given suspended sentences.
However, Luxembourg's state prosecutor lodged an appeal to overturn the verdicts, as did Deltour and Halet in a bid to have their sentences annulled.
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Consequently, all three men are back in court on Monday and the trial is expected to last until 21 December.
Parliament's GUE/NGL group had previously organised a letter of support to the whistleblowers signed by more than 100 MEPs.
Speaking from Luxembourg, GUE/NGL group MEP Stelios Kouloglou - who is a substitute on Parliament's committee of inquiry into money laundering, tax avoidance and tax evasion (PANA) - said, "It's a shame. These people are helping countries to fight corruption and to be better at collecting taxes. They should be rewarded instead of being sent to jail."
Meanwhile, fellow MEPs from the PANA committee, Fabio De Masi and Matt Carthy, have also lent their support to the whistleblowers via video, with committee Vice-Chair De Masi saying, "According to Oxfam, four of the world's top ten tax haven are in the EU. There is enough dirty laundry to be cleaned.
"Halet, Deltour and Perrin clearly acted in the public interest. The number of dubious tax rulings has actually increased dramatically since the LuxLeaks. This shows how urgently we need more decisive action against tax dumping in the EU," the German MEP argued.
Matt Carthy also offered his full solidarity with the three men on trial.
He said, "Luxleaks prompted an inquiry in the European Parliament but we can see that the political response has been totally insufficient.
"The fact that Jean-Claude Juncker - who oversaw this industrial-scale tax avoidance as Prime Minister in Luxembourg - remains the President of the European Commission shows just how insufficient the response at the political level has been," he added.
Dutch MEP Dennis De Jong, who as Co-President of the European Parliament's intergroup on integrity, transparency, corruption and organised crime (ITCO), has sought to prioritise the status of whistleblower protection in its upcoming EU anti-corruption report, said:
"The Luxembourg trial shows that we need European legislation to effectively protect whistleblowers in member states.
"As a first step, I have written an own-initiative report on the protection of whistleblowers relating to the protection of the financial interests of the EU. I call upon the Commission to take swift action after the adoption of this report by the Parliament which is due in February 2017," he said.