Hundreds of lawyers are demonstrating outside Parliament against reforms to the legal aid system.
The DCA criticised strikers for causing disruption to the courts
They say plans to pay per case, instead of per hour, will drive solicitors away and deprive people of representation in courts in England and Wales.
The government insists reforms are needed to cut spending, up by more than a third to £2bn in the last decade.
A spokeswoman for the Department for Constitutional Affairs (DCA) said the protest was "unprofessional".
The protest comes ahead of three days of unofficial industrial action in courts in England and Wales.
From April, lawyers will be paid per case, instead of per hour, and next year, defence solicitors' firms will have to bid for legal aid contracts.
Lawyers say the reforms will drive down standards and price solicitors out of the professions, leaving legal aids 'deserts' in parts of the country.
Those affected will include people needing legal help to settle housing and employment disputes and defendants in complex criminal cases, they say.
Protest organiser Julian Hayes said: "It is going to be impossible for firms to operate with the cuts that are being brought in.
"So there will be a vast reduction in the number of firms undertaking it, in fact a number of firms will go out of business.
"And individuals will not be able to seek advice, assistance and representation."
Lord Chancellor Lord Falconer told BBC Radio 4's The World At One: "If we really are serious about providing advice to the most needy then we've got to make sure the money we provide actually goes to the front line.
"Less fragmented firms, bigger firms, closer to where the work is being done, will provide more advice to people."
While there may be fewer firms doing the work, he said, the number of lawyers involved is unlikely to fall.
A DCA spokeswoman said: "Any disruption to the courts is against the public interest and is lamentable when risked by the professionals who earn their living there," she said.
"Victims of crime who may have been summoned as witnesses as well as their own defendant clients will not thank them.
"It is unprofessional and irresponsible for solicitors to abandon their court responsibilities to go on a rally."
Meanwhile, the Public and Commercial Services Union (PCS) is preparing to co-ordinate action with the defence solicitors in protest at what it describes as a funding crisis in the justice system.
More than 17,000 PCS members working in courts have been working to rule since the end of last year.
The dispute is due to intensify at the end of this month as a below-inflation pay award from the Department for Constitutional Affairs comes into effect.