Polling stations closed at noon (1000 GMT) after only a few hours, as most Swiss vote in advance by post or online.
Early projections from some of Switzerland's 26 cantons hinted that the country's largest party, the populist right-wing anti-immigration Swiss People's Party (SVP), had made gains and could possibly tip the scale in parliament towards a slight centre-right majority.
The expected shift comes as surging numbers of migrants and refugees moving through Europe have heightened the focus on the issue in Switzerland, even though the wealthy Alpine nation has yet to be significantly affected by the crisis.
But the latest survey from the gfs.Bern polling institute showed that 48 percent of those questioned thought migration was the most important issue facing the country.
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"I think there is a lot at stake, not only when it comes to the reception of refugees, but also the entire problem of the large numbers on the move," Colette Morel, a 69-year-old retired teacher, told AFP as she cast her ballot in the central canton of Fribourg.
"We need people (in parliament) who are prepared to discus the matter," she said, but declined to say who she had voted for.
SVP, which has previously sparked outcry with campaign posters such as one showing one of three white sheep kicking a black sheep off the Swiss flag, has in its national push this year been less provocative, resorting mainly to posters featuring pictures of candidates with the slogan "Stay free".
But some regional divisions of the party have gone much further, with the Vaud youth unit putting out a poster featuring a caricature of a jihadist, wearing an EU armband, preparing to decapitate a bound and gagged blond woman wearing a Swiss flag tank top, flanked by the caption: "Keep your head on your shoulders".
Nine per cent of those questioned in the survey said they were most concerned about Switzerland's relationship with the European Union, which was badly hit by a narrow Swiss popular vote in February 2014, championed by the SVP, in favour of restricting immigration from the bloc.
The centre-right Liberals, in third place, were also seen gaining ground, according to early projections, helping to move the pendulum in parliament towards the right.