Lagos state Gov. Akinwunmi Ambode addressed thousands of supporters at the event, organized by Nigeria's ruling All Progressives Congress party.Violence broke out after members of a transport union stormed the campaign venue, said Lagos police spokesman Chike Oti. The violence was directed at supporters of a top transport union official, MC Oluomo (real name Musiliu Akinsanya), who was among those stabbed at the rally, according to police. Police said the union official and others were discharged following treatment at a hospital.Video from the rally posted online shows men brandishing machetes as people cower and run for safety. Gunshots can be heard.Three journalists covering the event were injured, a Lagos-based journalists' union said in a statement on Wednesday.It called for a media boycott of political rallies by parties who cannot provide "minimum security" for journalists covering their campaigns.Police said 16 people were arrested following an investigation. Tuesday's incident has sparked fears of election-related violence as Nigeria begins a heated campaign season leading up to a general election in February.