MANILA, Philippines—Philippine press organizations demanded justice after reporters were among 21 people murdered Monday in what could probably the biggest group of journalists killed in one single incident.

Local authorities said at least 13 media people were killed in the massacre in Maguindanao province.

“It is incumbent on government to give swift justice to the victims. This is a direct challenge to our efforts to strengthen democracy in this country,” said Jaime Espina of the National Union of Journalists of the Philippines.

National Press Club of the Philippines president Benny Antiporda said President Gloria Macapagal-Arroyo had to prove she could deliver justice for the victims.

“The government needs to do something about this culture of impunity,” Antiporda said, referring to the widespread belief that the government had failed to prosecute those who attacked journalists in the past.

One journalist who narrowly avoided the massacre, Joseph Jubelag, editor of the Mindanao Bulletin, a local weekly, said he was originally meant to be part of the convoy.

But Jubelag said he decided not to go on when he was told by some soldiers that mysterious men had visited the guesthouse where the reporters were staying, asking for the names of the journalists accompanying the Mangudadatu clan.

“I was alarmed. I decided finally not to proceed,” he said.

The New York-based Committee to Protect Journalists said in a report this year that the Philippines was among several countries where media workers were murdered with impunity.

It ranked the situation in the Philippines as the sixth worst in the world, ahead even of war-torn Afghanistan. It said 65 journalists had been killed in the Philippines since 1992.