The incident occurred as motorists seeking to leave Gibraltar had to wait as long as four hours, the Royal Gibraltar Police said in statement Wednesday.
"A small number of missiles" were reportedly thrown at officers of the Guardia Civil from the motorcycle lane where cross-border workers were unable to return home, police said.
"It came after insults and abusive comments were directed at the Guardia Civil," the statement said.
The RGP said it had not been able to substantiate the report.
No one was reported injured in the incident.