The left-wing duo of Venezuelan leader Hugo Chavez and new Bolivian President Evo Morales will confront European and Latin American heads of state this morning with their controversial brand of South American socialism.
Both countries have riled governments from Brazil to Spain - Venezuela by throwing an Andean trade group into chaos and Bolivia by nationalising its oil and gas sector and accusing foreign energy giants of breaking its laws.
Mr Morales, a former coca farmer making his first presidential trip to Europe, relished the chance to challenge his region's former colonial powers and Bolivia's present heavyweight neighbour Brazil for their "pillaging" of his country.
"I am sure that it must be a novelty for you to see a president who comes from the battles of trade unionism, of social justice and the struggle for indigenous rights," he told reporters as he began a news conference yesterday.
"I am sure that the issue of nationalising the oil and gas resources is much more ... worrying for some countries," said Mr Morales who swept to power last December on pledges to use Bolivia's resources to help fight deep poverty.
Mr Chavez arrived later last night to the cheers of a few dozen supporters before Friday's summit in Vienna that will bring together nearly 60 heads of state and ministers from Europe, Latin America and the Caribbean.
The ex-paratrooper, who once led a failed coup before winning power in Venezuela through the ballot box, was also due to attend a rally along with a daughter of former Latin American revolutionary Ernesto "Che" Guevara.
Some of the other early summit arrivals spelt out concerns at Venezuela and Bolivia's partnership that is developing into a new political axis in South America.
Mexican President Vicente Fox warned they risked isolation from other countries in the region that are seeking to attract more foreign investment to boost growth, jobs and the kind of welfare gains that have eluded Latin America for decades.