A dozen Occupy Houston protesters were arrested Thursday in the fourth largest U.S. city for blocking public traffic. Hundreds of protesters met at a downtown park in the afternoon and began a march. Some protesters sat down in the middle of an intersection and blocked the traffic, according to the local TV channel KHOU. Houston police, including some on horseback, surrounded the protesters and ordered them to leave. When they refused, they were arrested, according to the report. About 13 protesters were arrested and will be charged with obstructing a public roadway, a Class B misdemeanor, the police said. The arrests in Houston came when protesters in Washington, Chicago, New York and many other U.S. cities took to the streets on Thursday to join the nationwide \"Day of Action\" to mark the movement\'s two-month anniversary. Some protests in the cities were also arrested. Last week, six people were arrested in the Tranquillity Park in downtown Houston after protesters placed a tarp over electrical equipment during a rainstorm. City officials considered the tarp a tent and said the protesters violated the city\'s ordinance against structures in public parks.