Five men were killed while they were drinking and playing dominoes inside a bar in El Salvador’s capital, San Salvador, on Sunday.
A sixth, who worked at a junkyard opposite, was shot dead as he fled on hearing the gunfire.
Police said three men were involved in the attack and that one man had been arrested in connection with the shooting.
In total, violence claimed 13 deaths across El Salvador on Sunday.
The spike in homicides comes a month after online newspaper El Faro alleged that the government of President Nayib Bukele had granted favours to imprisoned street gang leaders in exchange for a reduction in violence and backing at the polls.
In an interview with BBC Central America correspondent Will Grant last month, El Salvador’s Director of Prisons Osiris Luna Meza denied any such deal had been done.
El Salvador has one of the highest murder rates in the world, many of them gang related.
But in the first half of the year, the number of homicides fell by 62.8% compared to the same period in 2019, according to official figures.
President Bukele, who took office in June 2019, claims much of the credit for that drop.
He was elected on a promise to drive down El Salvador’s crime figures. His “territorial control plan” consists of concentrating security forces in areas worst hit by gangs.
Since he took office, there have been more than 30 days in which no murder was reported. Mr Bukele, who is a keen user of social media, has celebrated those days on Twitter.
On Sunday, he was quick to post on Twitter, but this time writing that he had convened a meeting of his security cabinet to “tackle the spike in homicides”.
He said he wanted to see “concrete results” from police.
Police said the man they had detained after Sunday’s attack on the bar was a gang member but they have not yet commented on a possible motive.
Local media said the bar owner was among the victims, whose ages ranged from 19 to 63.
Witnesses said the attackers had entered the bar and opened fire at close range without uttering a word.