Protesters gathered outside a £633m incinerator have stopped lorries from making the first deliveries of household waste for test burning.
Work on the plant near junction 12 of the M5 in Gloucestershire began in 2016 despite nearly a decade of protests.
The tests will take place before the full opening later in the summer.
Extinction Rebellion is calling for the waste to be checked for any recyclables on the grounds that burning recyclable waste is illegal.
Campaigner Katerina Hasapopolous said: "We're here to prevent a crime from being committed.
"Waste regulations of 2011 clearly state that you're not supposed to burn recyclables, compostables or re-useable materials and we suspect that is going to occur here today.
"We'll be stopping any rubbish trucks coming in and asking to see proof of what they are hauling."
The protesters have asked for the lorries to be opened up and checked but no permission has yet been given by the owners, Urbaser.
Waste will only come from Gloucester and Stroud councils, while other councils are set to continue using landfill for another week.
'Not a significant risk'
While the plant has now begun accepting waste it is not known how many tonnes are needed for a viable test to take place.
The partial opening of the plant coincides with a national study into the effect of incinerators on health.
The Public Health England (PHE) commissioned report concluded it is not possible to rule out all impacts on public health.
The Environment Agency has responsibility for overseeing the safe operation of the facility and will be monitoring air quality.
Gloucestershire County Council says the incinerator will provide electricity to power the equivalent of 25,000 homes, reduce carbon emissions compared to landfill and save taxpayers £100m.