The SNP has been branded "totalitarian" by one of its former deputy leaders.
Jim Sillars claimed the Nationalists were now the "most leadership-controlled party in the UK".
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He also criticised the party's backbenchers for not speaking out against its leaders, saying that there was the "astonishing spectacle" over "many years now of no rebellion against leadership policy and opinion".
Mr Sillars, who was SNP MP for Govan from 1988 to 1992, made the attack in a column in Holyrood magazine.
In it, he said: "If I did not know better, I would easily believe the leaders had been schooled in the old communist party, where the top, the elite, made the decisions and the rest fell into step automatically, with not a word of dissent.
"Totalitarian would be a fair description of Scotland's majority party."
Mr Sillars said it was "not possible" for all the SNP's MSPs to "be in total agreement with Salmond, Sturgeon and Swinney, yet no-one has dared tell them to get lost".
He added that "another unusual feature" of the SNP parliamentary party at Holyrood was there was no backbench policy groups looking at various important issues and getting experts involved in advising them.
Mr Sillars said at Westminster it was "taken for granted" that backbench MPs would form groups on areas such as defence, welfare or transport, with the intention of contributing ideas to internal debates on party policy.
While he claimed the Nationalists at Holyrood had "significant research capacity available", he added: "The evidence is that no serious work has been done.
"Those willing to be told to shut up seem happy to wait until the leadership issue edicts and statements and follow whatever line is laid down for them."
Mr Sillars went on to claim SNP MSPs "line up like soldiers on parade, emitting only one loyal cry: 'ours not to reason why"'.
He questioned: "Was there not one of them who, when confronted with the single police force, gave some thought to the issue of civil liberties, and the potential danger of too concentrated a power in the coercive force of the state, and backed that thought with a challenge to the Government?"
The former MP said: "There is a virtue in loyalty, but if loyalty is taken to a point where an MSP, or even worse, a whole group of them, cannot balance loyalty to party with loyalty to principle and the interests of the people, then it becomes dumb loyalty; and that leads inevitably to an intellectually dumb party."
There has been some internal opposition in the SNP from those opposed to ending the party's opposition to Nato membership.
Nationalist defence spokesman Angus Robertson will ask delegates at the party conference in October to back the new pro-Nato stance with the support of First Minister Alex Salmond.
But Mr Robertson's resolution faces opposition from a group led by MSP Jamie Hepburn, who has tabled an amendment urging the SNP conference to maintain its policy "that Scotland should not remain a member of Nato" because it "continues to be a nuclear weapons-based alliance".