(CNN) -- Occupy protesters succeeded in shutting down overnight operations at California's port of Oakland on Monday night after a day of similar protests in several other U.S. cities.
The protests in Oakland have "disrupted workers trying to get to work and impaired the port's ability to operate," port spokesman Issac Kos-Read told CNN. Craig Merrilees, a spokesman for the International Longshore and Warehouse Union, said the port has told its members not to report for work for the overnight shift because of the mass of protesters at the port.
Earlier Monday, the port authority said in a statement there were "some delays of truck traffic" but said the port remained operational.
On a normal night, several hundred people would be working the graveyard shift, Kos-Read said. Day shifts involve several thousand, he said. He said the protests have cost workers their wages, cost the city and region some revenue and could cause shipping firms to divert vessels to other ports.
"What has this accomplished?" he asked. "This is disrupting the 99%"
Oakland has been a flashpoint of the Occupy movement since October, when police used tear gas to break up demonstrators who refused to leave downtown. One demonstrator, a Marine veteran of the war in Iraq, suffered a skull fracture after being hit with a police projectile, according to a veteran's group; police said they acted after the crowd threw paint and other objects at officers.
The ILWU -- which represents 15,000 dockworkers -- has distanced itself from the protest movement. The union "shares the Occupy movement's concerns about the future of the middle class and corporate abuses," ILWU President Robert McEllrath wrote to locals last week -- but he urged the movement to stay out of its dispute with the port of Longview, Washington, and warned against "outside groups attempting to co-opt our struggle in order to advance a broader agenda."
Monday's demonstrations also took place in Los Angeles, Seattle, Houston and Portland, Oregon. Organizers said the goal was to shut down ports in an effort to "disrupt the economic machine that benefits the wealthiest individuals and corporations."
In Houston, police arrested 20 protesters after dozens of police on foot and on horseback confronted a somewhat larger group of Occupy protesters who blocked an interstate on-ramp, authorities said.
Groups of up to six protesters lay down on the pavement and interlocked arms and legs, while a larger group stood near them yelling protest slogans. Officers set up barricades to cordon off protesters in an attempt to free the ramp for traffic. The majority of the protesters could be seen moving behind the barricades, with a few exceptions, including those who had lain down.
Police handcuffed some protesters and led them to a police vehicle. Six face felony charges of using criminal instruments to block a public roadway, said Houston Police Department spokesman Victor Senties.
In Long Beach, California, protests caused isolated traffic delays but did not hinder port operations, according to Police Chief Jim McDonnell.
"Two arrests were made -- one for resisting and one for failure to disperse, as well as a number of traffic citations issued to pedestrians," he said.
About 80 protesters demonstrated outside the gate of San Diego's port, but caused no disruption, port spokesman Ron Powell said.
"They were there at a time when we really didn't have a lot of truck traffic coming in and out," he said.
Four people who sat down in the road were arrested, he said. San Diego police did not immediately return a telephone call seeking information on the arrests.
A spokesman for the port in Portland said the protests had partially shut down the port there. In addition to the West Coast port blockades, demonstrators in Salt Lake City and Denver said they were planning to disrupt operations of Walmart distribution facilities. About 40 to 50 people protested at the Denver facility, CNN affiliate KCNC reported.
The demonstrations were part of a nationwide day of protest called in the aftermath of efforts by cities across the country -- including New York, Boston and Oakland -- to clear demonstrators from encampments they had set up in public parks and other locations.
"We are occupying the ports as part of a day of action, boycott and march for full legalization and good jobs for all to draw attention to and protest the criminal system of concentrated wealth that depends on local and global exploitation of working people, and the denial of workers' rights to organize for decent pay, working conditions and benefits, in disregard for the environment and the health and safety of surrounding communities," organizers said on their website.
The port protesters are focusing on terminals owned by SSA Marine, saying it is owed by the Goldman Sachs investment firm, which they argue exemplifies corporate greed and is anti-union.
SSA Senior Vice President Bob Watters disputed the protesters' claims, saying Goldman Sachs owns less than 3% of an investment fund that has a minority stake in the company. He also said the company is the largest employer of International Longshore and Warehouse Union members on the West Coast.
In a letter to members sent last month, International Longshore and Warehouse Union president Robert McEllrath said the organization shares Occupy protesters concerns about what they consider corporate abuses, but he said the union was not sanctioning any shutdown.
Protest organizers said on their website that they were acting independently of organized labor only because the unions are "constrained under reactionary, anti-union federal legislation."
Some port workers are also against the planned blockade.
"I'm just barely getting on my feet again after two years, and now I gotta go a day without pay while somebody else has something to say that I'm not really sure is relevant to the cause," trucker Chuck Baca told CNN affiliate KGO.
Port officials say shutting down their facilities will only cost workers and their communities wages and tax revenue.
"Protesters wanted to send a message to the 1% but they are impacting the 99%," said Portland port spokesman Josh Thomas. The stoppage is resulting in "lost shifts, lost wages and delays," he said.
Port of San Diego board chairman Scott Peters issued an open letter to the community on Sunday asking that protesters not disrupt work.
"The Port of San Diego is made up of working people with families who serve the public each day by helping to bring in goods that are important to the people of the San Diego region," Peters wrote.
"They are the 99%, the gardeners, the maintenance workers, the dock workers, the Harbor Police officers, the office workers, the environmental workers -- all working to improve the quality of life in San Diego Bay and on its surrounding lands," he said. "It is these people who would be hurt by a blockade of our Port."
CNN's Greg Morrison, Amanda Watts, Shawn Nottingham and Sara Weisfeldt contributed to this report.