Community Land Scotland (CLS), which includes the owners of Eigg, Assynt and Gigha, said just 432 landowners accounted for half of Scotland's privately owned land, vast acres which were "at the mercy of the few". It said its "constructive and realistic" suggestions would allow for more diverse ownership.
But landowners denounced the proposals as divisive, destructive and absurd, saying they would have a "seriously detrimental" effect on the rural economy.
CLS made its proposals in evidence to the Scottish Government on the Community Empowerment Bill consultation.
It proposes non-crofting communities should enjoy an extended right to buy, similar to their crofting counterparts. Currently they are limited to a right of first refusal, if the owner decides to sell.
It says in circumstances where a request to purchase has been registered "and the triggered discussion, mediation or negotiation has failed", ministers should be able to give the green light to a community buyout, if it is in the public interest and if it furthers sustainable development.
CLS repeats its call for a land agency to be created to help in this process, but breaks new ground with two other proposals.
It wants a "new provision to require the approval of Scottish Ministers to the purchase of the most substantial areas of land most often seen in the sale of large estates, the threshold to be determined by regulation".
It also wants a "new provision for Scottish Ministers to have regard in their considerations and actions to the achievement of greater equality and diversity in the ownership of land in Scotland, including through community ownership".
David Cameron, the Harris-based chairman of CLS, said the Community Empowerment Bill proposed to alter the Land Reform Act to make it more effective.
He said: "At the heart of the issue of land reform sits the fact that too much of Scotland's land is owned by too few people. These people control vast acres and the development, or not, of that land is totally at the mercy of the few.
"As the debate for more land reform grows it is increasingly clear securing greater diversity in ownership is an objective more and more widely shared. To achieve that requires concrete legislative ideas. Our suggestions for how to bring about more diverse ownership are constructive and realistic."
But Douglas McAdam, chief executive of landowners' body Scottish Land and Estates, said: "When there are so many people and organisations working together to deliver real benefit to rural Scotland, the championing of the enforced sale of property and land businesses is a backward step - particularly since the First Minister himself has said that it is a route the Scottish Government did not want to pursue.
"The suggestion the state should interfere or have some control in an open property market is absurd. This would have a negative impact on the property market and send out the wrong message about Scotland being a place to do business. It could also have the effect of paralysing the many amicable transfers of land that happen across Scotland.
"Such a measure would be a threat to land businesses of any size and is completely out of step with the forward thinking requir-ed to benefit rural Scotland."
He said there already was legislation that provided communities with the first option to buy when land was put on the market by a willing seller and Scottish Land and Estates continued to support this system.