The Ecology and Population (Ecopop) group first circulated a petition that garnered 120,700 certified signatures, thus easily passing the 100,000-threshold needed for the proposed law to be put to a referendum.
If Ecopop's initiative clears other administrative and legal hurdles, it will be put to a national referendum, likely in 2015.
The group, which claims to be opposed to all forms of xenophobia and racism, insists Switzerland must limit immigration to avoid urbanization and to preserve its agricultural land and breathtaking nature.
Using scientific arguments, Ecopop is meanwhile likely to alienate its usual green bedfellows on the left and could instead seduce the anti-immigration populist right, political analysts said.
Ecopop, which bases its ideas heavily on the theories of US biologist Paul Ehrlich, famous for his controversial 1968 book "The Population Bomb", wants the Swiss government to commit to keeping population growth linked to migration below 0.2 percent annually.
This, it says, is "a level that is compatible with the sustainable preservation of natural resources."
"Switzerland currently has one of the densest populations on the planet, with 480 inhabitants per square kilometre in 'Mittelland'," or central Switzerland, Ecopop leader Andreas Thommen told AFP, insisting "this development is not at all sustainable in the long-term."
Switzerland, a country of some eight million people, counted 1.8 million foreigners at the end of August, which is 3 percent more than a year earlier, according to official statistics.
Ecopop blames a 2007 change in law making it easier for European Union citizens to settle in the country -- 1.77 million of Switzerland's foreign residents are from the bloc, with Italians, Germans and Portugese citizens accounting for the greatest numbers.
Etienne Piguet, a demography expert at Switzerland's Neuchâtel University, told AFP the country had indeed experienced "extraordinarily strong immigration in recent years."
But while Switzerland had faced some of the ecological problems brought up by Ecopop, he warned against "directly linking population numbers (and immigration) with such problems".
According to an online poll carried out recently by daily 20Minuten of 7,653 Internet users, 75 percent of Swiss voters support the initiative while 20 percent are opposed and five percent undecided.