"Just 2.5 months after collection began, 120,000 signatures for the SVP initiative 'against mass immigration' have been collected," said the Swiss People's Party in a statement.
Under Switzerland's direct democracy rules, any individual can push for a referendum on condition that he or she collects more than 100,000 from eligible voters to support the cause within 18 months.
The SVP said it would now begin the process of verifying the collected signatures with the aim of filing its initiative at the beginning of next year.
According to the initiative's draft text, the party wants Switzerland to "manage the immigration of foreigners in an autonomous manner".
In addition, it requires the country to impose a quota on the annual numbers of migrants admitted.
Any international treaties contravening these requirements "should be renegotiated and adapted within a deadline of three years".
Switzerland used to accept only a specific number of migrants annually. But after signing a deal with the European Union, citizens from the bloc are now free to reside in the country, provided that they are financially self-sufficient.
If accepted by the population, the SVP's initiative would therefore have an impact on the free movement of people treaty with the EU.
Migration is the biggest issue of the October 23rd elections in Switzerland, according to opinion polls.
The SVP had launched its anti-migration initiative in tandem with its campaign for the federal elections, with posters in train stations and city centres depicting the legs of men in suits marching across the Swiss flag, bearing the slogan "That's enough. Stop mass migration".
At the end of August 31, 2011, foreigners living in Switzerland numbered 1.751 million, making up 22.3 percent of the country's 7.9 million-strong population.