Danish prosecutors said an event attended by the crown prince was the main target for an alleged plot by four terrorists who wanted to retaliate for Danish newspapers publishing cartoons of the Prophet Mohammed.
The suspects were accused in a Copenhagen court of planning a shooting spree at Jyllands-Posten newspaper in Denmark in December 2010.
Prosecutor Henrik Plaehn said there was no proof that Prince Frederik was singled out but "there are things that point to the event itself being a specific target".
The men pleaded not guilty over accusations of a "Mumbai-style" terror plot and rejected claims that they had planned to kill several people at the offices of the paper.
State security police (PET) linked the men to a militant Islamist group with ties to international terrorist networks.
"It is our perception that an unknown number of people were to be killed by shooting," chief prosecutor Gyrithe Ulrich told TV2 N similar shooting spree took place in Mumbai in 2008, when 10 Pakistani gunmen killed 166 people in three days.
The men on trial are Mounir Ben Mohamed Dhahri, a Tunisian, and three Swedish citizens - Lebanese-born Munir Awad, Swedish-born Omar Abdalla Aboelazm, and Sahbi Ben Mohamed Zalouti, of Tunisian origin. They were arrested in the suburbs of Copenhagen in December 2010 on terrorism-related charges. Police said the attack was imminent "within days".
Jyllands-Posten was the first to issue a daily edition of the paper featuring a dozen cartoons caricaturing Islam and the Prophet Muhammad in 2005, sparking outrage against Denmark and riots in Muslim countries.
The trial is expected to last two months.
In 2001, a Danish court detained a Somali man for breaking into the home of a Danish cartoonist who had ridiculed the Prophet.
Last year, France's best known satirical magazine, Charlie Hebdo, was attacked following the announcement that the publication's latest issue would feature Mohammed as editor in chief.
The magazine's front cover showed an image of Mohammed saying "100 lashes if you are not dying of laughter".
The newspaper's office in Paris was firebombed and its website hacked.