The whole city shut down and residents expressed fears of a new military takeover as news spread that gunmen had fired on Mr Argana's vehicle as he drove to work yesterday morning. He was hit by at least 10 bullets in the head and body. Tension rose as troops and police poured into the streets to maintain order when the Vice-President's supporters gathered at the murder scene.
There was little doubt that the killing was the result of apower struggle among politicians of the long-ruling Colorado party, closely tied with the military that ruled for 35 years until 1989 under the dictatorAlfredo Stroessner. Mr Argana had been foreign minister and head of the Supreme Court under General Stroessner.
Mr Argana was in line to take over from the President, Raul Cubas, if the leader were to be removed. Congress began impeachment proceedings against the President last week, accusing him of abuse of power.
President Cubas has ordered the release of General Lino Oviedo, who had tried to launch a military coup against then president Juan Carlos Wasmosy in 1996 and was later sentenced to 10 years' jail. General Oviedo's release late last year led to a split within the Colorado party between factions supporting either Mr Cubas or Mr Argana. Paraguayans knew the split was deep but only yesterday did they realise it was deadly.
Mr Argana felt he had been robbed of the party's presidential nomination, and in effect the presidency, in 1993 as a result of a military-inspired intrigue to install Mr Wasmosy as the candidate. Mr Wasmosy won the presidency and ruled until last year.
Mr Argana felt robbed again last year. He was defeated in the party primary for presidential candidate by General Oviedo, who planned to run his campaign from jail but was later barred from the race. Mr Cubas was then given the candidacy, won the presidential election last May and freed General Oviedo later in the year.
Mr Argana's role as Vice-President was becoming increasingly uncomfortable after Congress, despite a Colorado party majority in both chambers, voted to start impeachment proceedings against Mr Cubas. Mr Argana had persuaded his supporters within Congress, although from the same party as the President, to vote in favour of the impeachment move. Mr Cubas's supporters were furious.
There was no indication yesterday who the gunmen were but the authorities blocked all land borders and launched intensive airport checks to stop them leaving the country.