About 15,000 people stage street demonstration in Seoul . South Korea says it will not allow North Korean leader to visit for summit talks with U.S. President Barack Obama on April 27 and 28. Riot police fired water cannons and sprayed fire extinguishers to repel them .
About 15,000 people stage street demonstration in Seoul . Rally turned violent after some protesters tried to drag away police buses . Riot police fired water cannons and sprayed fire extinguishers to repel them .