Workers on the 57km Gotthard base tunnel, Europe's largest construction site, have downed tools for 24 hours over a collective labour agreement, according to unions.
However, constructors described the action as an unconstitutional blockade by the unions and said not one worker had gone on strike.
Industrial action began on the New Railway Links through the Alps (NRLA) at 8pm on Friday when, according to the Unia, Syna and OCST unions, workers on the late shift in the sites of Faido, Bodio and Amsteg went on strike.
The unions said workers had blocked the entrances to the mining galleries with their machines and were protesting against a lack of contracts in the construction sector.
At 4am on Saturday miners at the NRLA site of Sedrun also began industrial action, according to the unions, who said the workers "were confirming their determination to fight against the diktats of the Swiss Construction Association and for a new national covering contract".
They said this initial warning strike, which by Unia and Syna estimates involved a total of around 600 workers, would continue until 8pm on Saturday.
The aim of the strike, they said, was to negotiate and push through a new national covering contract for the more than 80,000 construction workers in Switzerland, the old one having been terminated by the Construction Association in September.
They added central to the strike was the issue of hours not worked, an area where employers were demanding more flexibility.
The Construction Association on the other hand didn't mince its words in condemning the action, which it described as an "unconstitutional blockade".
No one was striking, the association said, it was in fact union officials and third parties who were blocking the construction sites and preventing those who wanted to work from entering the tunnel.
A spokesman for Switzerland's largest construction firm Implenia said not a single one of its 1,500 workers had taken part in the strike and that "to avoid any trouble, management had decided to suspend construction and not force entry".
The Construction Association warned the unions that they were putting further negotiations at risk and that costly blockades would not result in new national covering contracts.
They also pointed out that workers on the Gotthard received above-average wages and generous benefits payments.
Renzo Simoni, head of the NRLA construction group Alptransit Gotthard, "the NRLA was one again being misused for political purposes".
For the unions, however, the industrial action at the Gotthard is only the start in their struggle for new contracts. Further strikes are planned for Monday in Geneva, Neuchâtel and Bern and for November 1 in Zurich.
The next round of negotiations with the Construction Association is scheduled for November 5.
The 57km Gotthard base tunnel, due to open in 2017, will overtake the 34.6km Lötschberg rail tunnel as the longest rail tunnel in Switzerland and will become the longest tunnel in the world.
The Lötschberg was officially opened by Transport Minister Moritz Leuenberger in June 2007 and is set to be fully operational in December.
The Lötschberg, which runs from Frutigen in the Kander valley to Raron in canton Valais, and the Gotthard base rail tunnel being built between Erstfeld and Bodio are part of an overall plan by the Swiss government to create a more efficient rail network and help ease the heavy burden of transalpine traffic.
The policy was approved by Swiss voters in 1998.
swissinfo with agencies
The Gotthard - Europe's alpine axis
An interactive presentationend of infobox
Gotthard base tunnel
With a length of 57 km, the new rail tunnel, part of the New Rail Links through the Alps project, will be the longest of its kind in the world.
On September 1, 2007, of the total of 153.5 km of tunnels, galleries and passages of the Gotthard Base Tunnel, 104.7 km or 68.2% had been excavated.
The Lötschberg base tunnel, which runs 34.6 km, was inaugurated on June 16. Total costs are around SFr4.3 billion.
When the two projects were started, the total cost of the two new lines was put at SFr14.7 billion. It's now estimated that it could exceed SFr24 billion.