The monument blocks the view of scrolling illuminated messages - some of them about human rights - displayed on the building by American diplomats.
It consists of 138 black flags with a white star.
Havana says it symbolises the people who have died as the result of violent acts against Cuba since its 1959 revolution - more than 3,400 of them.
Vigil
The monument was unveiled during a ceremony organised by the government to honour victims of what it considers US-sponsored violence against Cuba.
"They are white stars over a black background, representing the light of a people that are in pain and mourning for their children and families," said professor Carlos Alberto Cremata, standing next to President Castro.
Mr Cremata is the son of the co-pilot of a Cuban airliner which was bombed in 1976, killing all 73 people aboard.
After the flags were raised, Cubans began a 24-hour vigil in front of the US mission, holding posters with the faces of the dead.
The US mission in Havana began displaying the electronic messages in January.
President Castro described the scrolling messages as a gross provocation, and soon turned what had once been the car park of the building into a major construction site.
Huge cranes were brought in and teams of builders worked there non-stop.
But no-one would say what was being built - not even President Castro - until the monument was unveiled on Monday.
The propaganda war between the US and Cuba is nothing new - but correspondents say this is an escalation.
Already Cuba has put up scores of posters in the capital caricaturing President Bush as both a fascist and a vampire.
The two countries have not had diplomatic links for 45 years.
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