TOKYO?A group of Japanese cartoonists on Monday protested against a Tokyo government plan to tighten rules on sexual images of minors in comics, animation and computer game software.

Cartoonists, including prominent artists Machiko Satonaka and Tetsuya Chiba, held a news conference to voice their opposition to the plan before the Tokyo metropolitan assembly votes on the proposal on Friday.

"This could violate freedom of expression," they said in a joint statement to assembly members of the ruling Democratic Party of Japan.

Japan's capital has proposed to call on publishers to exercise greater restraint on sales of any cartoon works that feature sexual depictions of characters that people would assume to be underage.

The local government ordinance, which would be effective only in the capital, would also designate any comics showing sexual violence as "noxious publications" and tighten restrictions for under-aged readers.

Japan is a major global source of child pornography. Although producing and distributing it is illegal, possession is not criminalized, and images of "non-existent" underage characters in manga comics are legal.

The Japanese central government has separately launched a working team, made up of police, education, justice and other senior officials, to tighten measures against child pornography.

The body has been tasked with drawing up new measures by June, which could include a crackdown on online child porn and the blocking of sites, public education campaigns, and steps to find and help the victims.