Taipei, Taiwan ? Tossing bottles and rocks, hundreds of Taiwanese protesters were blasted by police water cannons Saturday after a rally demanding an independent investigation of the bizarre shooting that injured President Chen Shui-bian one day before his narrow re-election.

The rowdy protests have become a weekend tradition since Chen won the March 10 vote and opposition candidate Lien Chan challenged the results. Lien has insinuated that Chen might have staged the unexplained shooting, and he wants a special task force to investigate the event. So far, police have no suspects or solid leads.

Saturday’s rally began peacefully, with about 50,000 people gathering in front of the presidential office to listen to Lien and other politicians give speeches. They also signed a petition demanding a referendum on whether the shooting commission should be created.

But after the rally ended, about 1,000 protesters gathered at a barricade in front of the presidential office. They began taunting police guarding the barrier, and some threw bottles and rocks. Others heaved sheets of metal on the barricade and threatened to scale it.

Police blasted the crowd with water cannons when the protesters tore down scaffolding from a rally stage and tried to use the metal structure to ram the police barricade. The protesters fought back, hitting police shields with metal rods, flag poles and umbrellas.

About four hours later, more than 1,000 police formed a thick column and began slowly pushing the crowd out of the area. Some of the protesters threw scraps of metal and bottles at police as they retreated. One demonstrator tossed a gasoline bomb, but no one was injured.