The men - aged between 18 and 27 and mostly of Moroccan descent - were arrested after the 2 November murder of film director Theo van Gogh. They are accused of belonging to an Islamist network dubbed the Hofstad.
"We are dealing with a very active radical group," public prosecutor Koos Plooy told a pre-trial hearing in Rotterdam on Monday.
The dozen have been charged with belonging to a criminal group and conspiring to murder with "terrorist intent". Two are charged with the attempted murder of police officers.
"The group plotted some very serious crimes including
murder, possession of arms, issuing threats and causing
explosions, all following extremist convictions," Plooy said.
Prosecutors said Muhammad Buyari, a Dutch-Moroccan
charged with shooting and stabbing Van Gogh as he cycled to work in Amsterdam three months ago, had close links with the group.
The Hofstad group, prosecutors say, were on a mission to target Dutch public figures known to be critical of Islam.
Plooy said copies of the letter found pinned to Van Gogh's chest with a knife that threatened leading Dutch politicians were found on computers owned by several of the suspects.
Politician Ayaan Hirsi Ali is said
to have been a possible target
He said the arrest of the 12 men prevented other attacks.
The men are accused of threatening to kill Ayaan Hirsi Ali,
the Somali-born Dutch politician with whom Van Gogh made a film that accused Islam of condoning violence against women.
They are also accused of threats against the anti-immigration populist Geert Wilders, Amsterdam's Jewish mayor Job Cohen and his deputy Ahmed Abu Talib.
Two of the men are also charged with the attempted murder of police officers at whom they threw a grenade when the officers tried to arrest them in The Hague a week after the Van Gogh murder.
Plooy said police had found a handbook on religious killing
at the home of The Hague suspects. At the home of another
suspect, they found a letter welcoming the reader into "our
terrorist organisation" and promising "paradise" as a reward.
Only one of the 12 suspects, Moroccan-born 26-year-old Ahmed Hamdi, was in court.
Hamdi sat with his arms crossed and did not react as Plooy addressed the court.
Plooy said Hamdi attended religious meetings at Bouyeri's house which the Dutch secret service said drew about 20 people.
"We all know that Hamdi will be found not guilty"
He said all the suspects had exercised their right to remain silent as soon as questions became detailed. Investigators still had a mountain of evidence to sift through, including checking email accounts and transcribing wiretaps, he said.
Hamdi's defence lawyer Michiel Pestman called the proceedings a show trial but said his client would eventually be shown to be innocent. "My appearance here gives a veneer of legitimacy to this trial, while we all know that Hamdi will be found not guilty," he said.