A 13-year-old boy injured in police firing died in a hospital here on Monday, taking the toll to 23 in the unrest that has engulfed the Kashmir Valley after a militant commander's killing.Shahid Gulzar was injured in firing by police on Saturday in a south Kashmir village of Shopian district. He succumbed to his injuries at the SMHS Hospital, a police officer said.Police sources said more violence erupted in parts of the south and north Kashmir on Monday.They said an angry mob set ablaze a police picket in Lassipora in Pulwama district in south Kashmir. Another police camp was set afire in north Kashmir's Sopore town, the sources said.The Kashmir Valley has been on an edge since Friday evening when senior militant commander Burhan Wani was killed by security forces.Curfew continued in parts of the valley for the third day amid fears of more violence. Apart from a policeman killed by a mob, 22 civilians, dominantly the young, were killed in firing on street protests.