ZURICH (Reuters) - Swiss environmentalists, tapping into unease about population levels in their landlocked nation at the heart of Europe, on Friday presented the government with 120,000 signatures to force a referendum on limiting immigration.

Under the Swiss system of direct democracy, 100,000 signatures need to be collected to put a cause to a national vote. Referenda take place up to four times a year after the government sets dates for the voting.

Ecopop, an organization which campaigns on environmental and population issues, on Friday handed in boxes containing the signatures supporting their initiative "Stop overpopulation to safeguard natural resources".

The initiative calls for an annual limit on Swiss population growth through immigration of 0.2 percent. It also demands that the government invest at least 10 percent of its international aid budget into measures to support family planning abroad.

"The pressure on land, nature and the countryside is considerable and quality of life is continuously deteriorating due to a lack of living space," said Philippe Roch, a former director of the environment department and member of Ecopop.

The imitative reflects growing concern about overcrowding among residents of Switzerland, where the population reached the 8 million mark this year - a rise of more than 140 percent since 1990. In March, voters backed a proposal to severely limit the building of second homes after proponents argued rampant over development risked destroying Switzerland's natural beauty.

The right-wing Swiss People's Party (SVP) - which has long blamed rising rents and crowded transport on an influx of foreigners - handed in an initiative earlier this year to "stop mass immigration" by reimposing quotas on foreigners from the European Union.

Switzerland's government has urged voters not to back the initiative saying it would bring Switzerland into conflict with its treaty obligations with the European Union.

The number of foreigners living in Switzerland rose 3 percent in the year to August to 1.8 million.

Business lobby group Economiesuisse said Ecopop's initiative would compromise Switzerland's relationship with the EU and impose "senseless rules" on Swiss development aid.

(Reporting by Caroline Copley)