Two supporters of Ecuador President Rafael Correa were killed on Monday after a lone man attacked an election rally with a large knife.
Four more were injured in the attack, two weeks before a presidential vote Mr Correa is expected to win easily.
Police have arrested the man they think is responsible.
"Best wishes to the families of the Quininde victims," Mr Correa said on Twitter. "The video of the attack is spine-chilling."
Local TV showed footage of the attack on the Alianza Pais rally in Quininde, a town in the western Esmeraldas province.
President Correa, a firm ally of socialist Venezuelan leader Hugo Chavez, is expected to win the February 17 vote with ease.
Polls give him between 50 and 60 per cent of votes, at least 30 percentage points ahead of his nearest rival, former banker Guillermo Lasso.
The progressive president is very popular in urban shanty towns and rural areas, where millions of poor Ecuadorians have seen their livelihoods improved thanks to record spending on roads, hospitals and schools.
The US-trained economist has been at odds with foreign investors since taking office in 2007.
He rattled Wall Street when he defaulted on £2 billion on foreign bonds in 2008, and two years later forced foreign oil companies to sign deals granting the state a larger share of their revenues.