Retired general Juan Pereda Asbun, 79, was "in a lamentable state," speaking incoherently and incapable of physical coordination when he was taken into custody on Tuesday, prosecutor Francisca Rivero said in an interview with the PAT television station.

"We asked that the former president undergo an expert examination to determine his state of mind," she said.

"The laboratory established that Pereda Asbun was under the effects of cocaine."

An air force general, Pereda led a military coup in 1978 that overthrew his mentor, the then dictator Hugo Banzer, who had ruled the country since 1971.

Pereda was only in power for four months, but he ushered in a long period of political instability.

Rivero said police arrested Pereda after neighbours complained that he was engaging in obscene acts in the street.

Police searched his car and found a pack of cigarettes that tests showed contained drugs, she said.