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.
His lawyer will not say if the heavily tattooed man will testify in his own defense. He faces up to a year in jail.
Schwaiger said his client decided to try the case rather than agreeing to a settlement.
On Thursday, both sides showed police witnesses a published photograph subpoenaed from The Press Democrat. Cairo's lawyer said it depicted officers hitting Cairo on the head with a picket sign. But an officer testified it captured him deflecting a blow from Cairo.
The trial will also focus on Cairo's role in a Dec. 3 protest outside the Santa Rosa Veterans Memorial building, where Ravitch was hosting a re-election fundraiser.
Prosecutors said Cairo got in a tussle with officers when they tried to close a door to prevent his group of about 50 people from disrupting the event. Santa Rosa police Sgt. Eric Litchfield testified the outnumbered officers were never able to shut the door and had to stand in the threshold while Cairo and others yelled at them with bullhorns.
Schwaiger suggested to jurors it is wrong that "now the very person he was protesting is asking you to find him guilty of crimes."
UPDATED: Please read and follow our commenting policy: