In an interview with AFP this week, Mas said any attempt to take away powers from Catalonia to stop such a vote from taking place would likely create a "strong" backlash in a region led by a government backed by a majority-separatist parliament.
But he also sought to ease fears about secessionist plans in the northeastern region at a time of global jitters.
Mas, a 61-year-old former economist, said he thought the trial was merely taking place because of the "big success" of the symbolic, non-binding referendum held in November 2014 when he was still president of Catalonia.
He and two former members of his government are accused of severe civil disobedience and misconduct for holding the vote after it was banned by Spain's Constitutional Court, which had declared it illegal.
More than 80 percent of those who cast their ballot in the vote did so for independence - although just 2.3 million people out of a total of 6.3 million eligible voters took part.
"These 2.3 million people who went to vote exceeded all forecasts, irritating the Spanish government which was accused of being soft and sparking an over-reaction," he said.
"Essential and basic democratic rights such as citizen participation and the right to freedom of expression are being violated."
Mas, Catalan president from 2010 to 2016, has now been replaced by Carles Puigdemont, the former mayor of Girona.
Resolutely pro-independence, he has pledged to hold a referendum in September - a binding one this time, with or without Madrid's consent.
But how exactly it will go ahead is unclear, as the central government has vowed never to allow an act that would risk the unity of Spain.
Last week, reports emerged that Madrid was considering drastic measures to stop a vote, such as closing schools where polling booths could be set up or taking control of the police, which is normally managed by regional authorities.
The government neither confirmed nor publicly denied the reports.
"Those who are considering interfering in Catalan autonomy should think twice, as they don't know what the Catalan reaction will be," Mas said.
"Depending on how aggressive (the interference) will be, the reaction of Catalan society could also be very strong. It will be democratic and peaceful, but it will be very strong."
Mas acknowledged that there was currently a lot of "commotion" in the world at a time when the region's pro-independence government is trying to take steps towards splitting from Spain.
The rise of far-right parties in Europe, Britain's shock decision to exit the European Union, or Donald Trump's election to the US presidency, pledging more protectionism, have sent jitters round the world, with people fearful of more instability.
But Mas sought to allay fears over Catalonia's intentions.
"We're in favour of the European project, the European Union, the euro, of welcoming refugees," he said.
Mas said that Catalonia's pro-independence authorities may be up against the state and its institutions, but still have the power to call people onto the streets.
Millions have protested over the past years to demand greater autonomy and outright independence, and some 40,000 people showed up at the start of Mas's trial in Barcelona on Monday.
"Europe and the world must understand that this is not a movement organised by Catalan politicians, it's a movement channelled by some Catalan politicians but rooted in the people."
Still, the Catalans are very divided over the issue - 44.9 percent want independence while 45.1 percent don't, according to a recent poll conducted by a Catalan public institute.
A large majority, however, wants a referendum to have their say, once and for all, the survey said.