"I hope that everybody recognises now that there is a hybrid war and one day it's Poland, the other day it's Denmark, and next week it will probably be somewhere else that we see sabotage or we see drones flying," Frederiksen said.
Wednesday's summit comes after a spate of troubling drone incidents at Danish airports and military bases over the last week. Ahead of the meeting, a special radar system was set up at Copenhagen airport to help keep watch.
Unidentified drones forced the closure of the airfield a week ago, causing major disruptions.
France, Germany, the Netherlands, Sweden and the UK also sent aircraft, ships and air defence systems to Denmark ahead of the talks.Ukraine's armed forces have dispatched a mission to the Nordic country for joint exercises, sharing its expertise on combating Russian drones.
While the Danish authorities have not identified those believed to be responsible, Frederiksen said: "There is only one country that are willing to threaten us and it is Russia and therefore we need a very strong answer back."
"I want us to rearm. I want us to buy more capabilities. I want us to innovate more, for example on drones," she said.
"When I look at Europe today, I think we are in the most difficult and dangerous situation since the end of the Second World War."
Russia is the focus of Wednesday's summit, where discussions are expected to centre on how to prepare Europe to fend off Russian aggression by 2030, especially as the United States turns its focus on security concerns in Asia and elsewhere.
Ukraine is also high on the agenda, and Ukrainian President Volodymyr Zelenskiy is due later on Wednesday to address the EU leaders via videolink.
Leaders and intelligence services believe that Russia could mount an assault elsewhere in Europe in three to five years, and that President Vladimir Putin is intent on testing NATO as doubts swirl about US President Donald Trump's commitment to the organisation.
A new proposal to use frozen Russian assets to help Ukraine will be discussed, as well as its prospects of joining the EU. Trump has ruled out NATO membership for the country, the best security guarantee available.
The leaders will gather formally on Thursday for talks focused on security, trafficking and migration.
