Union officials promise the demonstrations will culminate on Black Friday with demonstrations or walkouts at hundreds of stores across the country.
Wal-Mart's U.S. division employs about 1.3 million workers.
"Wal-Mart is doing everything in its power to attempt to silence our voice," said Colby Harris, who works at a Wal-Mart store in Lancaster, Tex. "But nothing_not even this baseless unfair labor practice charge_will stop us from speaking out."
Dan Fogleman, a Wal-Mart spokesman, described the latest tactics from the group as "another exaggerated union campaign."
"Many of these ongoing tactics by the UFCW are unlawful and we will protect our associates and customers," he said. If the disruptions violate the law "we will take the appropriate action to hold them accountable."
Kelly P. Kissel in Little Rock contributed to this report.