The attack was recorded by a security camera in an ATM vestibule in Ponta Grossa, Brazil.
Soldiers patrolled the Angolan capital on Sunday as Angolans watched to see if plans for a Monday mass protest against the 31-year rule of President Jose Eduardo dos Santos would materialise.
Since last month, rumours have been circulating on the Internet of north Africa-style protests scheduled to begin on March 7.
Many have dismissed the anonymous call to protest as a charade, but the ruling party, the Popular Movement for the Liberation of Angola (MPLA), has treated it as a credible threat, organising large pro-go0vernment demonstrations Saturday in Luanda and several other cities in a bid to show its strength.
Political analyst Justino Pinto de Andrade, an economist at Catholic University in Luanda, called the protest plans a "joke" -- but said the MPLA's reaction showed the ruling party was nervous after 35 years in power.
"This anonymous protest, at night, spontaneous, is a provocation that doesn't make any sense," Andrade told AFP.
But, he added, that the MPLA's statements in the press and Saturday's pro-government rallies "have shown a certain anxiety on the part of the MPLA, which is afraid."
There were fewer people than normal on the streets of Luanda and in beach-side restaurants Sunday, though an upcoming carnival holiday due to start on Tuesday likely contributed to the quiet. Military also patrolled the streets, though armed soldiers are a common sight in Luanda.
Elias Isaac, head of non-profit Open Society's Angola office, said the country faced problems similar to those that sparked the unrest in north Africa, but was unlikely to return to violence so soon after its 27-year civil war, which ended in 2002.
"There are enough ingredients in Angola for people to easily connect with what's happening in north Africa, especially in the social and political arenas," he said.
"More than 75 percent of the population lives in poverty. People have no access to water or electricity. There is unemployment, the health and education systems function poorly."
But, he added, "There have only been eight years of peace here, and the people, who lost everything, aren't ready to let go of the little they've acquired."
Investigative journalist Rafael Marques said regardless of whether the protests happened, the rumours were a warning of the potential for unrest in Angola, which vies with Nigeria for the title of Africa's top oil producer but has struggled to lift living standards for most of its people.
"What outrages people is that Angola is a rich country. The government knows well that the level of discontent is growing," Marques said.
"All this has created a debate on the idea of regime change," he added.
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