Defence solicitors withdrew their labour for the day from the custody courts at Edinburgh, Glasgow and Paisley sheriff courts.
Legal chiefs said the action strikes at the heart of access to justice in Scotland and is a sign of the concern and frustration being felt within the profession.
The escalating row centres on proposals contained in the Scottish Civil Justice Council and Criminal Legal Assistance Bill.
Lawyers are worried about plans for accused people with a disposable income of £68 a week or more to contribute to the cost of their defence in summary cases.
They are also concerned about moves for solicitors to collect the money themselves, instead of the Scottish Legal Aid Board, and say it will alter the relationship between lawyer and client.
Today, the Glasgow Bar Association (GBA) and Edinburgh Bar Association (EBA) confirmed they were boycotting the cities' custody courts, alongside colleagues at Paisley Sheriff Court.
It comes less than 10 days after solicitors stayed away from Edinburgh Sheriff Court's custody court in a day of protest action.
EBA vice-president Mark Harrower today said the Government was trying to force the changes through at a time when solicitors have reached "breaking point".
"Solicitors in Edinburgh are standing shoulder to shoulder with their colleagues in Glasgow and Paisley as the protest action spreads across courts in Scotland," he said.
"Lawyers are disappointed at the Justice Secretary's approach in recent days, which has been to give an indication in public that he is willing to negotiate with the Law Society and reach a sensible conclusion to this problem, whereas in fact he's refusing to budge.
"We feel he's seriously misjudged the sense of feeling amongst the profession, who've had to absorb repeated cuts in recent years.
"We feel we're left with no option but to withdraw our labour in the face of a Government which is taking a hard line.
"The strength of feeling about the manner in which these cuts are to be imposed upon us is unprecedented amongst the profession."
GBA president Bernadette Baxter said: "The Government would lead the public to believe that this is lawyers fighting about fees. It is not.
"Criminal defence agents across Scotland will continue to protest until or unless the Government amends the current proposed legislation.
"There have been no increases in fees in 20 years in Scotland for criminal defence agents, rather there have been repeated cuts. Defence agents across the country have borne that.
"This is the first time ever that national action has been taken by defence agents in Scotland. It has been brought about as a result of the Government introducing proposals which will decimate access to justice in Scotland.
"Lower pay than 20 years ago did not cause this protest action. Justice on the cheap did.
"The profession are united and Glasgow Bar Association supports wholeheartedly the protest action against changes that will affect some of the most vulnerable members of our society and those on low income."
Lawyers recently staged a protest against the changes outside Parliament, which was attended by Law Society of Scotland president Austin Lafferty.
Of the latest action, Mr Lafferty said today: "It is a sign of the concern and frustration within the profession that solicitors have felt forced to make this kind of protest.
"Solicitors working in legal aid have already borne a series of reforms and fee cuts in a bid to save money from the public purse.
"Many now see this latest package of proposals as simply a step too far and a risk to the legal aid system that Scotland needs in order to be a civilised, fair and just society.
"Solicitors care about their work and care about their clients. They also care about our system of justice.
"That is why we will continue to argue for a legal aid system that is fair, workable and protects the most vulnerable in our society."