The group issued photos of its members using ladders to scale a fence to enter the premises of the 45-year-old Beznau power plant in Döttingen, which it claims is “ridden with unacceptable deficiencies”.
It said participants included members from Switzerland, Germany, Austria, France, Liechtenstein, Italy, Poland, Hungary and Slovenia.
The group raised banners with the words, "The End" around various sectors of the plant.
By this action, Greenpeace said it wanted to avoid, three years after the catastrophe of Fukishima, Switzerland being hit by a similar disaster.
Aargau cantonal police confirmed the intrusion of the nuclear site, operated by the energy company Axpo.
Officials from the plant alerted police around 7am, police spokesmen said, although different precise times were cited.
Police estimated the number of activists at 30, far fewer than the number given by Greenpeace.
A defective safety switch triggered an automatic shutdown of the Beznau plant in November 2012 when steam escaped from a non-nuclear part of the plant.
After an interruption of about 24 hours, the plant was brought back on line.
Switzerland reacted swiftly to the nuclear disaster in Fukushima in 2011, with parliament deciding to phase out nuclear energy.
Under current plans the country's five reactors will be put out of action by 2034.
Beznau is due to be decommissioned in 2019, but that's not soon enough for environmentalists who have repeatedly called for its immediate closure.
Axpo, however, maintains that the plant, located on an island between the Aare River and a canal, is regularly upgraded with the latest technology.
On its website, the company said Beznau and its other nuclear plants "must satisfy the most stringent safety regulations and be cost-efficient on a long-term basis".
But Greenpeace's action raised questions about the plant's security.
"How could Greenpeace so easily storm the plant?" asked Zurich-based daily newspaper Blick on its website, prompting concerns about what terrorists could do.
Axpo spokesman Tobias Kistner told the tabloid that the plant is protected by a company guard and an alarm system.
Police were alerted at 6.55am and responded within minutes, cantonal police spokesman Roland Pfister told Blick, who added: "we have experience with activists".
Pfister said officers would be checking the identity of the Greenpeace protestors and further action would be determined by the local prosecutor's office.