Puigdemont successfully avoided extradition to Spain when a German court refused to send him back on charges of rebellion past year.
Puigdemont said in Berlin that the trial represented "a stress test for Spanish democracy".
In his opening remarks to the trial, which is being transmitted live on national television, a lawyer defending two of the accused said they had the right to seek independence for their region. The government says they are being judged exclusively in line with the rule of law.
Notably absent from the list is former Catalan president Carles Puigdemont, who is in self-imposed exile in Belgium and thus can not be tried in Spain.
Raul Romeva, one of nine defendants jailed without bail since late 2017 facing the charge of rebellion, has made clear his view that it should end only with an acquittal.
The 2017 attempt to declare Catalonia's independence followed a referendum that had been carried out in defiance of a judicial ban, and angered much of the rest of Spain.
But there was shock at home as well as overseas when police used batons and rubber bullets on protesters on the day of the vote.
The Geneva-based International Commission of Jurists, a human rights group of judges and lawyers, said the case risked restricting rights and could set a precedent.
The trial, arguably Spain's most important in four decades of democracy, started as the future of Prime Minister Pedro Sanchez's minority government hinged on a last-minute change of position by Catalan pro-independence parties to back his 2019 budget.
"If there is no budget, it won't be the Catalan pro-independence parties' fault, it will be because you think you will benefit from an early election", said Ferran Bel, from the Catalan PDeCat party, addressing the government.
Sanchez's socialist PSOE party holds less than a quarter of seats in parliament and needed support from the smaller regional parties to get the budget passed.
Supporters of the defendants, who face up to 25 years in prison if convicted, say the trial is political.
In an early morning tweet, Sanchez had said he expected both right-wing and pro-independence lawmakers to vote against his budget.
"We are not demanding anything that isn't democratic, that wouldn't be done via ballot boxes", he said.
"Both want the same thing: that Catalonia be at war with itself and that Spain be at war with itself".
Secessionists have called on Catalans to briefly stop work at midday on Tuesday in protest against the trial, and to join a rally in Barcelona in the evening. Another big demonstration is planned for Saturday and a general strike on February 21.
After imposing direct rule on Catalonia in October 2017 in response to the independence declaration, the previous conservative government called regional elections to curb the independence movement. But the move backfired as the secessionists retained a slim majority in the Catalan assembly.
The trial is expected to last at least three months, with a final ruling being delivered in June or later.