I suppose you're thinking of 'Blood Upon the Grass' which was written ba Glasgow singer/songwriter Adam McNaughtan, not Ewan MacColl. This is what I have:

BLOOD UPON THE GRASS
(Adam McNaughtan)

September the eleventh
In Nineteen seventy-three
Scores of people perished
In a vile machine-gun spree
Santiago stadium
Became a place to kill
But a Scottish football team
Will grace it with their skill
And there's blood upon the grass
And there's blood upon the grass

Will you go there, Alan Rough
Will you play there, Tom Forsyth
Where so many folk met early
The Grim Reaper with his scythe
These people weren't terrorists
They weren't Party hacks
But some were maybe goalkeepers
And some were centre backs
And there's blood upon the grass
And there's blood upon the grass

Victor Jara played guitar
As he was led into the ground
Then they broke all of his fingers
So his strings no more could sound
Still he kept on singing
Songs of freedom, songs of peace
And though they gunned him down
His message doesn't cease
And there's blood upon the grass
And there's blood upon the grass

Will you go there, Archie Gemmill
Will you play there, Andy Gray
Will it trouble you to hear the voice
Of Victor Jara say
Somos cinquo mille -
We are five thousand in this place
And Scottish football helps to hide
The Junta's dark disgrace
And there's blood upon the grass
And there's blood upon the grass

Do you stand upon the terracing
At Ibrox or Parkhead
Do you cheer the Saints in black and white
The Dons in flaming red
All those who died in Chile
Were people of your kind
Let's tell the football bosses
That it's time they changed their mind
Before there's blood upon their hands^^

This is what it was all about:

[1977:] A Labour MP yesterday slammed the SFA [Scottish Football Association] for insisting that the proposed international in Chile this summer should go ahead. A new row broke out several days ago after SFA secretary Willie Allan stated that any player who refused to play in the match would face disciplinary action. Mr. Norman Buchan, MP for West Renfrewshire, said that the SFA didn't appear to comprehend what happened in the Santiago stadium where the game is to take place. It had been used as a concentration camp and was the scene of mass murder and torture. (Sunday Mail, 9 January)

[1977:] About 70 per cent of Scottish professional footballers voted in favour of the national team playing Chile in June. Only ten per cent were opposed. (Glasgow Herald, 22 January)
Officials of the SFA today refused to meet a delegation of three former prisoners of the Chilean military regime who called at their headquarters in Glasgow. All three were held prisoner in the Santiago stadium, where the match is scheduled to be played. Mr. Willie Allan was unable to meet them because he was attending a meeting. Mr. Ernie Walker declared that he could see no point in meeting the delegation. (Glasgow Herald, 8 March)

[1983:] In their preparations for the 1978 World Cup in Argentina, the Scottish football team undertook to play Chile in the Santiago Stadium. During the military coup of 1973 the stadium had been used for the internment of five thousand people. It had seen, in particular, the mutilation and murder of the singer and songwriter, Victor Jara. It was with that in mind, rather than any developed theory about politics and sport, that I joined in opposing the match with this song. (Notes Adam McNaughtan, 'WordsWordsWords')

[1989:] Salvador Allende was a popular, democratic socialist, and the mood of the people who supported him was reflected in the flourishing New Chilean Song Movement. The movement had been growing throughout the sixties, and was a modern version of all that Woody Guthrie and Pete Seeger had tried to achieve decades earlier. The singers would meet at a peña, an artists' co-op [...]. The best-known Chilean singer was Victor Jara, who had gradually switched his style in the sixties from personal to political songs [...] and he had become a regular performer at Allende's rallies or May Day demonstrations. After the victory of Allende's Popular Unity Party, Jara became actively involved in campaigns to retain his popularity in the face of violence from the right wing, and increasingly disruptive strike campaigns by managerial classes and others opposed to his policies. [After the military coup in September, 1973, Jara was] arrested, tortured, and held with thousands of others in the Santiago Stadium, and [sang] Popular Unity's hymn Venceremos before he died. (Denselow, When the Music's Over 117ff)

[1993:] [In September, 1973,] General Pinochet, with the assistance of the CIA and the ITT Corporation, took over the government of Chile, bombing the presidential palace of elected socialist Salvador Allende, and murdering him. Victor was singing for students at the university when the whole area was surrounded. All within were taken prisoner and marched to a large indoor soccer stadium, Estadio Chile. For three days it was a scene of horror. Torture, executions. An officer thought he recognized Victor, pointed at him with a questioning look and motioning as if strumming a guitar. Victor nodded. He was seized, taken to the center of the stadium and told to put his hands on a table. While his friends watched in horror, rifle butts beat his hands to bloody pulp.
"All right, sing for us now, you ---," shouted the officer. Victor staggered to his feet, faced the stands. "Companeros, let's sing for el commandante." Waving his bloody stumps, he led them in the anthem of Salvador Allende's Popular Unity Party. Other prisoners hesitantly joined in.
RAT-TAT-TAT-TAT.
The guards sprayed him and the stands with machine guns. (Seeger, Where Have All the Flowers Gone 102)

Then there was the German politician who travelled to Chile in 1977 and came back telling us, 'In the summer life in the stadium is quite agreeable'. - Susanne