Solicitors and probation officers in England and Wales are staging joint action against changes introduced by the Ministry of Justice.
Members of the probation officers' union Napo are going on strike over "dangerous" government plans to outsource 70% of the service.
Criminal defence solicitors walked out for two days from midday in protest against cuts in legal aid fees.
The London Criminal Courts Solicitors' Association (LCCSA) said police stations would also see delays due to a reduced number of solicitors working.
Nicola Hill, president of the LCCSA, said of the joint action:
As the government dismantles the criminal justice system, risks are being taken which threaten public safety and the right to a fair defence.
The MoJ is taking a gamble on outsourcing the supervision of the most persistent offenders on release from short-term prison sentences.
Meanwhile, as the justice secretary scrimps on legal aid, the risk of the innocent going to jail is set to become a nightmarish reality.
The LCCSA has estimated that two-thirds of legal aid firms will close when the cuts are introduced - a total of 1,000 across England and Wales.
BBC News - Solicitors and probation workers strike over plans.