Hundreds of independent truckers went on strike at several U.S. ports Monday, the Associated Press reports. The action slowed the movement of cargo containers that can hold everything from furniture to electronics to frozen food. It was unclear exactly how many truckers went on strike nationwide. Truckers had called for a strike from Monday to Sunday. Nationwide independent truckers, who get paid by the load, are upset over wages, fuel costs and anti-union laws. The truckers also want to be able to form unions and collectively bargain for better contracts from shipping and trucking companies that hire them. About 200 independent truckers demonstrated at Port Newark, one of the nation's busiest container ports. Dozens of independent truckers walked picket lines in South Carolina outside the Port of Charleston, the nation's fourth-busiest container port, the news agency reports. Owner-operators of trucks are considered independent businesspeople and are forbidden by federal price-fixing laws from negotiating or talking with employers together.