"With a first-round exit from Euro 2004 beckoning, it could have been a disastrous night for the Portuguese."
Andrei,
"The Portuguese" is correct in that sentence, "Portuguese" is a noun that means "the people from Portugal".
It may be perhaps confusing sometimes since the language is also called "Portuguese", and the adjective to describe things or people coming from Portugal is also "Portuguese".
The difference is precisely the article that appears in the first sentence of the article you posted.
Compare the following:
"It could have been a disastrous night for the Portuguese." (the people, noun)
"I don't speak Portuguese." (the language, noun)
"Portuguese wines are exported to many countries." (adjective premodifying 'wines')
I hope that'll help you see the difference.
You also say "It would be fine to say the people of Lisbon. I think the entire population or rather the nation of Portugese could be uncountable."
It is very common to refer to a country, or to the people of a country in general, in such cases:
"Argentina won the world cup in 1978 and 1986."
"The Irish are excellent rugbiers."
"The Japanese were on the losing side in WW II."
Miriam