Bolivian President Evo Morales has claimed that there have been several assassination plots against him in the past, but the results of investigations have never been released, causing some Bolivians and the international community to doubt their veracity.

He has a turbulent relationship with the US. Last year, he expelled the US Drug Enforcement Administration (DEA) from his country. He also claimed that groups associated with the US government were planning to assassinate him. Washington has denied all such charges.

Morales is the leader of the political party Movimiento al Socialismo (Movement for Socialism). The party was involved in social protests such as the gas conflict and the Cochabamba protests of 2000. It aims at giving more power to the country's indigenous and poor communities by means of land reforms and redistribution of gas wealth.

Police chief Victor Hugo Escobar said the three men, including Irishman Mike Dwyer, shot dead in Santa Cruz early on Thursday morning, were believed to be part of a conspiracy to kill Morales. Two other suspects were arrested.

Morales is Bolivia's first indigenous president and has faced strong opposition in relatively wealthy regions of the country, including Santa Cruz. The governor of Santa Cruz, Ruben Costas, a prominent critic of Morales, has voiced skepticism about the government's account.

The gang the government claims was plotting to kill the Bolivian president was also behind a dynamite attack on the residence of Roman Catholic Cardinal Julio Terrazas last week, government officials have alleged.

Morales has accused right-wing politicians and business leaders in Santa Cruz of organising violent protests there last year to try to destabilise his government. On Thursday, he said the rightist opposition wanted to "riddle us with bullets", referring to himself and the vice president.

The government said the group planned to attack the president, vice-president and other authorities and personalities, including Santa Cruz governor Ruben Costas, a leader of political opposition to Morales.

It was unclear why a group of alleged anti-Morales assassins would attack Costas, who is known to support the president's opponents controlling much of Bolivia's farm and natural gas wealth in the lowland east around Santa Cruz.

Costas has questioned the government's information, saying it was "mounting a show" to discredit the opposition. Santa Cruz and three other states have adopted measures seeking greater autonomy from Morales' central government.

Morales has accused Costas of fomenting anti-government violence after rioters in September seized state buildings to block a vote on a new constitution.

Eleven people died in the skirmishes, and a UN report found the president's political opponents responsible.