The decision comes one day after Russian President Vladimir Putin urged delaying the vote to create the conditions necessary for dialogue.
Ukrainian authorities said "anti-terror" operations would continue regardless of the rebels' decision.
Millions of ballot papers have been prepared for the vote.
"The referendum will take place on 11 May. We are getting ready, ballot papers are being printed, everything remains in force. Nothing will change, it will not be delayed," Russian news agency RIA Novosti quoted activists in the eastern region of Luhansk as saying.
A pro-Russian activist leader in Donetsk, Denis Pushilin, said the decision to press ahead with Sunday's plebiscite was unanimous.
The suggestion to postpone the vote may have come "from a person who indeed cares for the people of the south-east," he told the AP news agency, "but we are the bullhorn of the people".
Moscow has vowed to protect the rights of Ukraine's Russian-speaking population against what it calls an undemocratic government in Kiev.
Ukrainian authorities have rejected activist demands for greater autonomy and troops have been battling to regain official buildings occupied by rebels in the east.
The European Union warned of the dangers of pressing ahead with the vote.
"Such a vote could have no democratic legitimacy and would only further worsen the situation," a spokesperson for the EU foreign policy chief told reporters.
The separatists' resolution to hold the referendum comes as a Pew Research Center poll released on Thursday shows that a strong majority of Ukrainians want their country to remain unified, even in the largely Russian-speaking east.
On Wednesday, the White House said the "illegitimate, illegal" plebiscite should be cancelled rather than postponed.
The US and the European Union have imposed sanctions against several Russian individuals and businesses and threatened wider measures if Moscow interferes further in eastern Ukraine.
Sunday's planned referendum was seen as a potential trigger for that.
Ukrainian interim Prime Minister Arseniy Yatseniyuk dismissed Mr Putin's calls to delay the referendum as "hot air".
But in what appeared to be a further shift in Russian policy, Mr Putin also said on Wednesday that this month's presidential elections in Ukraine were a move "in the right direction".
His remarks came days after his spokesman said holding such an election would be absurd.
On Wednesday, President Putin also announced he was ordering Russian troops back from the Ukrainian border.
But one day after Mr Putin's comments, NATO said there was still no sign of any Russian troop withdrawal.
Unrest in the south and east of Ukraine has worsened since Russia annexed the Crimean peninsula in March.
That followed the ousting of Ukraine's pro-Russian President Viktor Yanukovych in February by pro-Western protesters.
Activists in Donetsk say the decision was unanimous.
Russia's President had suggested a delay might calm tensions in Ukraine, where heavily armed rebels have taken control in the city of Slaviansk - and their supporters occupy official buildings across the region.
But they argue that pressing ahead with the vote is the peoples' choice.
They respect President Putin - they say - but don't answer to him.
So on Sunday, those who turn out will face one question: do you support the declaration of independence, of the Donetsk People's Republic.
What that means - and what happens next - is unclear.
As for the government in Kiev, it's stressed again that this referendum is unconstitutional and its result will not be recognized.
Source: BBC News Europe