Police in Brazil arrested 19 people in an operation targeting supposed Flamengo football supporters allegedly planning to run amok during this week's Liberators Cup semi-final against Gremio at Rio de Janeiro's Maracana stadium, officials said.

The two Brazilian clubs have been drawn together in South America's premier continental club soccer tournament and police had been wary of the possibility of hooliganism.

More than 100 members of a WhatsApp group exchanged audio and video messages threatening to "kill cops, steal, cause damage and embarrass Gremio fans" during Wednesday's highly anticipated match between the rival clubs, police said in a statement.

They arrested 19 out of 27 people targeted.

State media said one person was shot dead and two others were wounded in the operation in a poor neighbourhood in the city's north.

Police declined to comment.

Flamengo, the most popular football club in Brazil, is hoping to secure a berth in South America's Liberators Cup final for only the second time in its history.