According to Oakland police, specialized units began monitoring the port at about 5:50 a.m. Police were attempting to keep protesters from blocking the entrances to the port.
Roughly six people have been cited for allegedly obstructing port activities, police spokeswoman Johnna Watson said.
The Port of Oakland Truckers Association said members voted to go forward with the work stoppage after meetings with Oakland Mayor Jean Quan and other leaders failed to produce the changes they are seeking.
Association spokesman Cesar Parra estimated that there were about 180 protesters at the port early Wednesday morning, and that the number had dwindled to roughly 60 as of about 10 a.m.
Among other demands, the truckers are asking for an extension on a Jan. 1 deadline to comply with new state regulations that require them to upgrade their trucks, and are seeking a "green emissions" payment to truckers to offset the cost.
The truckers' association said new trucks cost between $50,000 and $80,000 and many truck drivers are struggling to pay for the upgrades.
The group is also asking for a congestion fee of $50 per hour for truckers who end up spending more than two hours waiting in line to pick up a load at the port.
In a statement released earlier this week, Quan said that as many as 15 percent of the 6,000 registered truckers at the port haven't yet met the new state air quality regulations.
She said today's protest involves "a few hundred" among that group, and that the work stoppage could "hurt the other truck owners at the port who have already invested in upgrading their rigs."
Quan said the port, as well as state and regional agencies, have allocated $38 million to help port truckers cover the cost of air quality upgrades to their rigs.
"We must ensure that every truck that does business at the port continues to meet the emission standards set five years ago," the mayor said. "State and federal agencies have allowed multiple deadline extensions and are not offering more."
Truck drivers held a previous work stoppage at the port on Oct. 21. The truckers are independent contractors and are unable to form a union.