The case - filed in September 2020 against the 27 EU member states as well as Britain, Switzerland, Norway, Russia and Turkey - is the largest climate case ever to be heard by the European Court of Human Rights (ECHR) in Strasbourg.
A ruling in the case is expected in the first half of 2024.
The applicants argue inaction to tackle climate change threatens their rights including to life, physical and mental wellbeing.
"Due to heat extremes, I'm limited in how I exercise and how much time I can spend outdoors," 15-year-old applicant Andre Oliveira said outside court. "I'm forced to stay inside, I struggle to sleep and thanks to the weak climate policies of these governments, things are getting worse."
Backed by the British-based Global Legal Action Network (GLAN), the Portuguese applicants, aged between 11 and 24, want a legally-binding decision that would force states to act.
If the complaint is upheld, it could result in orders from national courts for governments to cut carbon dioxide emissions blamed for climate change faster than currently planned.
A lawyer for the Portuguese government told the court that the evidence provided failed to show the specific damages caused by climate change on the lives of the young applicants.
Gerry Liston, one of GLAN's lawyers, said: "Portugal stood up on behalf of all respondent states and claimed what the applicants had been describing was just a figment of their imagination and that's gaslighting."
'UNPRECEDENTED'
Greece, in a submission sent to the court before the hearing, said "effects of climate change, as recorded so far, do not seem to directly affect human life or human health", an argument another GLAN lawyer, Gearoid O'Cuinn, described as "climate denialism".
Responding to criticism, Petros Varelidis, from the Greek environment ministry, said the case had no legal grounds and highlighted Greece's ambitious climate legislation.
Last week, Greek Prime Minister Kyriakos Mitsotakis told the United Nations General Assembly the wildfires and floods that struck his country this summer led to deaths and destruction.
Applicant Oliveira previously told Reuters their goal was to force governments to "do what they promised they would do", referring to the 2015 Paris Agreement to cut emissions to limit global warming to 2 degrees Celsius and ideally 1.5C. Current policies would fail to meet either goal, according to the United Nations' Intergovernmental Panel on Climate Change.
Four of the six applicants are from the central Portuguese region of Leiria, where two wildfires killed more than 100 people in 2017.
A lawyer representing Britain said the consequences of climate change were "global" but that the protection of applicants' interests fell under Portugal's jurisdiction. He said the case should be rejected.
More than 80 lawyers represented the accused countries, while the applicants were represented by six lawyers.
Climate litigation is growing, both in Europe and beyond.
Last month, a judge in Montana, in the United States, handed a historic win to young plaintiffs in a climate change case. In addition to Wednesday's youth case, there are two other climate cases pending before the ECHR's Grand Chamber.
