RIO DE JANEIRO—Brazil’s government is still scrambling to patch together a team of weapons screeners for Olympics venues, this time using retired police officers with little experience operating X-ray machines, many of whom just arrived in the host city in recent days.

On the eve of the opening ceremonies, they are being trained in the use of metal detectors and other equipment and have yet to be posted at some venues, the government has said. Moreover, some are being taught to use the equipment by inexperienced, low-wage...