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 government said the reforms were necessary in each case.
Up to 14,000 people are expected to take part.
Magistrates' and crown courts in major cities including London, Manchester, Birmingham and Leeds are expected to be affected.
The London Criminal Courts Solicitors' Association (LCCSA) said police stations would also see delays due to a reduced number of solicitors working.
'Untried and untested'
It is the third time solicitors have staged a walkout, and Napo's second strike, but the two sets of workers have never joined forces to take action before.
They are due to gather at Westminster on Tuesday afternoon for a rally and will then march to the Ministry of Justice.
Under the ministry's plans, most probation work, supervising low and medium-risk offenders, would be outsourced to private firms and voluntary groups.
Napo general secretary Ian Lawrence said: "The government plans to outsource 70% of the probation service are untried and untested.
"It is a dangerous social experiment that we believe will lead to a reduction in rehabilitation and fragmented risk management, placing the public at risk."
He added that while it was a difficult decision for union members to take strike action, "the public needs to know how these changes will impact on risk management and ultimately public protection".
Justice minister Jeremy Wright said: "It is very disappointing that Napo has chosen to strike when we are making positive progress towards the implementation of these vital reforms.
"We are pleased that Napo, along with the other probation trade unions, has recently ratified a national agreement on staff transfer that will provide a series of protections for probation staff.
"We are therefore surprised that they have now chosen to adopt this unhelpful course of action."
Solicitors have warned that those accused of crimes could be at risk of miscarriages of justice if they do not get a "decent" lawyer, due to the legal aid cuts.
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.
Matt Foot, founder of the Justice Alliance, which is fighting against legal aid cuts, said: "This is the first day in history when solicitors and probation officers have taken joint action not to work the courts.
"They are united against a minister of justice who is listening to no one, not even government-funded reports."
Solicitors last took part in a walkout on 7 March, along with criminal barristers, which caused widespread disruption.
Last week, barristers called off industrial action after the government agreed to delay its proposed legal aid cuts until at least the summer of 2015.
Legal Aid minister Shailesh Vara said: "We've always said we want to do all that we can to help lawyers facing fee cuts.
"We've spoken at length with them and made changes to our initial plan as a result. But this government is dealing with an unprecedented financial challenge and the Ministry of Justice has no choice but to significantly reduce the amount it spends each year.
"Our final package does mean fee reductions, but we've staggered them to try and ease their impact."