It happened while Cairo was at the center of a "powder keg" of about 100 demonstrators who converged on a City Council meeting, prosecutor Chris Brown told jurors in opening statements.
Cairo became aggressive when officers told him and 55 others carrying the crosses to put them down before they could enter, he said.
Instead, Cairo began pushing and shoving police and eventually swung the sign at them, officers testified.
But Cairo's lawyer, Izaak Schwaiger, denied that was what happened. He said Cairo was part of a non-violent demonstration hoping to call attention to 56 officer-involved deaths in Sonoma County, including last year's Lopez killing.
Cairo tried to enter the public building when he was stopped by police. He said Cairo was targeted because he is a protest leader and because he's been openly critical of District Attorney Jill Ravitch, who is charged with deciding whether the deputy who killed Lopez, Erick Gelhaus, committed a crime.
"This is a case about the criminalization of political dissent," Schwaiger said.
The matter could become clarified by a video both sides are expected to show Friday that captures parts of the protest, which drew a police SWAT team and sheriff's helicopter.
The nearly 12-minute video depicts Cairo and other demonstrators assembling with wooden crosses in front of City Hall and marching silently to the door. A fight breaks out when police stop them from entering.
Protesters are eventually allowed to enter the City Council chambers, halting the night's meeting. Cairo addresses council members with a bullhorn for up to 10 minutes. He then is arrested as protesters leave the building.
Cairo, who has a long criminal record, was initially charged with a felony count of resisting an officer by force or violence. His charges were reduced to three misdemeanors — two counts of resisting and one count of disturbing the peace — after a preliminary hearing.