A broad coalition at the University of California formed a united front Thursday, joining in a protest that participants say will be the first of many opposing budget cuts across the 10-campus system. Students, faculty, staff and unionized labor workers on a one-day strike participated in organized class walkouts, picketing and teach-ins. Jorge Serrato, a senior at the Riverside campus, had declared the Riverside campus’s walkout a success by mid-afternoon. “The whole [protest] spot was completely flooded by students,” said Serrato, raising his voice over bongo drums and bullhorns in the background. Participants in the Riverside protest estimated that as many as 500 to 1,000 protesters attended rallies at peak times. Davis campus officials used a Web site to communicate the impact of the walkout, indicating that some professors had canceled classes and e-mailed students syllabuses and assignments. Officials at the University of California president’s office said the protests had caused “minimal” disruptions to classes. The demonstrations came in response to the university regents' approach to filling an $813 million budget gap, which they have addressed with a combination of furloughs and tuition hikes. If regents approve another tuition increase in November, tuition could go up by as much as 45 percent in a two-year period.