Le Pen could not find words harsh enough for the decision, describing it as a "veritable coup d'état" by "politically motivated magistrates", as well as a "political assassination" and an "attack on democracy".
Examining magistrates Claire Thépaut and Renaud Van Ruymbeke called for the seizure of two millions euros on 28 June, it emerged on Sunday.
The RN, like other French poilitical parties, was to receive payment of a state subsidy on Monday.
The judges reportedly feared the money would be used to pay off its substantial debts, leaving nothing to reimburse the European parliament if the fraud inquiry goes against it.
Although Le Pen said party would receive 4.5 million euros on Monday, she told the BFM TV channel the confiscation would "mean the death of the National Rally at the end of August" because it would be unable to pay its staff.
Accusing the magistrature of having a left-wing bias, Le Pen said the judges had violated the principle of presumption of innocence and "decided to assassinate France's leading opposition party".
"To justify such a seizure it has to be shown that it is the product of the alleged fraud," the RN's lawyer, David Dassa-Le Deist, said on Sunday. "But there is no possible connection between this public subsidy and the wrong alleged by the European Parliament."
There was no danger of any fines or reimbursement not being paid, he argued, since the party is to receive subsidies to the tune of 20 million euros between now and 2022.
The party, which was forced to take a loan from a Russian bank in 2014 because of difficulties raising loans in France, has launched an online appeal for donations.
The European Parliament is demanding seven million euros from the party because it says it paid full-time staff in France using money granted for assistants in Brussels.
As well as the party itself, nine MEPs, including Marine Le Pen and her father, Jean-Marie, are being investigated for alleged fraud, a charge they deny.