Legal arguments began Tuesday in Quebec Superior Court with those opposed to the law saying even police have refrained from using it.
But they said some of its provisions must be struck down immediately — before they're applied.
"If the law, one year later, is found to be unconstitutional, there are people who will have been arrested and judged while the law was unconstitutional," lawyer Giuseppe Sciortino told reporters.
Nearly 20 lawyers representing student federations, unions and other groups are trying to get parts of the law that pertain to public protest temporarily suspended.
Since Bill 78 came into force on May 18, police across the province have barely applied it, though it has provided fuel for protests and for opponents of the Charest government.
The motion being heard Tuesday is one of two legal challenges to the law. The second is to have Bill 78 declared invalid altogether and it will be heard at a later date, possibly next month.
Lawyers for the students and other groups say the provincial law is ambiguous and is an assault on protesters' right to assembly and free expression.
Sciortino told the judge earlier Tuesday that the Charest government chose not to respect student democracy by refusing to recognize their strike votes, leaving them no choice but to boycott their courses. Just under one-third of Quebec post-secondary students have voted in favour of striking.
The government argues that Bill 78 preserves other rights — like that of students who want to attend school — while also respecting free speech.
Quebec government lawyers will make arguments Wednesday before Justice Francois Rolland, chief justice of the Quebec Superior Court, who is hearing the case.
It's a case student leaders have said could find its way before the Supreme Court of Canada.
The temporary law is designed to expire after a year, anyway.
It lays out rules for demonstrations, such as ordering assemblies of more than 50 people to give eight hours' notice of the protest route and state the estimated duration of the event. It also sets hefty fines for people who contravene the law.
The government introduced the law as a way to cool down heated student protests, but it may even have backfired politically, as protests against the government have grown larger, attracted more diverse crowds, and spread to different cities.
Another lawyer arguing on behalf of students said police have demonstrated they have the tools they need to deal with protests. Thousands of protesters have been ticketed recently — under existing municipal bylaws and traffic-code violations, not Bill 78.
Felix-Antoine Michaud said if police haven't used Bill 78 yet, it's because they don't need it. There are also concerns the bill provides police an arbitrary power that allows them to declare a protest illegal if they don't like the itinerary or timing. It opens the door to possible abuse of power, he said.
"The law doesn't say what types are good or not, it only says that spontaneous protests are illegal if there isn't a notice eight hours prior," Michaud said.
By that definition, a popular weekly meeting of drummers called the tam-tams, held every Sunday on Mount Royal, is illegal, he said. So was a spontaneous celebration in Shawinigan when the Cataractes won the Memorial Cup hockey tournament.
He gave the judge another example.
"So, if the Italians win (at the Euro 2012 soccer tournament) and thousands of fans celebrate, they haven't advised authorities in advance because they can't know if they will win, so they will find themselves committing an illegal act," Michaud asked.