Court and police interpreters in Birmingham protest
The Ministry of Justice (MoJ) outsourced interpretation work to private firm Applied Language Solutions (ALS) in a bid to save £18m a year.
But many freelance interpreters will not work for ALS, saying their pay will drop and the new system is failing.
About 40 protesters are outside West Midlands police headquarters and crown and magistrates' courts.
The MoJ said the contractor had made a marked improvement after being told to take "urgent steps".
Capita, which owns ALS, said it had been awarded the contract to address "weaknesses, lack of transparency and disproportionate costs" in the previous system.
One of the protesters, on a loud hailer, led calls of "scrap the contract" and "end the contract".
'Period of transition'
Irina Jefremova, a Russian and Lithuanian translator, said her pay would dramatically fall if she worked for ALS.
She said: "Why would I want to earn a third of what I was paid before and be associated with random, bi-lingual unqualified people who work for ALS?"
A spokeswoman for the freelance interpreters claimed that since the new system was brought in several weeks ago, court cases were being adjourned across England and Wales costing "hundreds of thousands of pounds" because interpreters had not been supplied or unqualified or inexperienced workers sent.
"This policy has been a disaster for the courts, the police and the public," she said.
"The project was intended to cut costs but costs are in fact spiralling out of control.
"Not only that, but the company Applied Language Solutions is supplying unqualified and inexperienced interpreters wasting huge amounts of court time."
However, Capita denied these claims, saying ALS had 1,800 experienced, qualified translators, with more signing up daily.
A Capita spokeswoman added: "Inevitably there will be a period of transition as embedded but inefficient working practices are changed with the aim of achieving higher quality and more cost effective services."
The MoJ said there had been an "unacceptable number of problems" in the first weeks of the contract but action had been taken.
"They have put measures in place to resolve these issues and we have already seen a marked improvement," a spokesperson said.
The MoJ added it was committed to ensuring the rights and needs of those who require interpreters were safeguarded, adding the new system would be monitored "on a daily basis".