The comments are likely to cause further strain in relations with Europe after weeks of tension.
Turks will decide whether to approve an executive presidency that would axe the role of premier and boost Erdogan's powers in the poll, seen as a crossroads in the modern history of Turkey.
From March 27 until April 9, Turks in Europe are able to vote in the referendum, including in Germany where there are 1.4 million votes up for grabs in a critical battleground.
Erdogan praised Turks abroad, who he said were voting in greater numbers compared with previous legislative elections in 2015.
"Let it be more and say 'yes' in the ballot boxes. God willing, give the necessary answer to those pushing fascist practices, those grandchildren of Nazism!" he said during a rally in the northern Turkish province of Rize.
The comments are likely to cause further strain in relations with Europe after weeks of tension which began after the Netherlands and Germany blocked Turkish ministers from speaking at rallies last month.
In response, Erdogan repeatedly compared some countries' actions with "Nazi practices", despite pleas from European leaders to tone down his language.
"Try to block our brothers and sisters from voting in Europe however much you like. Our siblings there will burst those ballot boxes in Europe with God's permission," he said.
In apparent reference to EU leaders deemed hostile to Ankara, he added: "We would not allow three, five European fascists to hurt this country's honour".
During several rallies in the aftermath of last July's failed coup, many Erdogan supporters have called for the death penalty in another element of alarm for Europe.
Erdogan again said he would approve such a call if parliament passed a law bringing back capital punishment after the measure was abolished in 2004 as part of Turkey's EU membership drive.
But he also said, if necessary, there could also be a second referendum on bringing capital punishment back.
Any return would see the end of Turkey's bid to join the bloc, Brussels has indicated, but Erdogan said this did not matter to him.