Protesters shout slogans during a march against the government's planned secrecy law in Tokyo November 21, 2013. REUTERS/Issei Kato

By Kiyoshi Takenaka

TOKYO (Reuters) - Thousands of people protested in Tokyo on Thursday against a proposed secrets act that critics say would stifle information on issues such as the Fukushima nuclear crisis.

The law, proposed by Prime Minister Shinzo Abe's government, would significantly broaden the definition of official secrets, which Abe says is vital for strengthening security cooperation with main ally the United States and other countries. Tough secrecy regulations before and during World War Two have long made such legislation taboo, but the law is expected to pass when it comes to a vote next week, given the comfortable majority the ruling coalition has in both houses of parliament.

"Without the right to know, democracy cannot exist," said Yasunari Fujimoto, from the Peace Forum citizen's group, who spoke at the protest in a park near parliament.

"If this law comes to pass, our constitution is nothing more than a scrap of paper."

Critics say the law would prevent journalists from investigating official mistakes, such as the collusion between regulators and utilities that contributed to the 2011 Fukushima nuclear disaster.

Anyone convicted of breaking the law could be jailed for up to five years.

Protesters packed into a 3,000-seat outdoor theatre in the park, with people standing in the aisles and spilling out into the park. Some held signs saying "Don't take away our freedom."

Organizers put the turnout at around 10,000.

Abe insists the law is also essential to his plan to set up a U.S.-style National Security Council.

Legal and media experts say the law is too broad and vague, making it impossible to predict what would come under its umbrella. The lack of an independent review process leaves wide latitude for abuse, they say.

"This law is absolutely unacceptable. We have a right to know everything," said Akio Hirose, a 54-year-old transport worker.

"After all, we are the voters."

(Writing by Elaine Lies; Editing by Robert Birsel)