PARIS (Reuters) - France's far-right National Front stepped up its anti-European Union rhetoric on Wednesday, days after toughening its stand against immigration, in an apparent bid to mobilize its core supporters ahead of Sunday's presidential election.

Opinion polls have for months predicted that National Front (FN) chief Marine Le Pen would attract enough votes to make it to the two-way runoff on May 7 but the gap with other contenders has narrowed, and the rhetoric of her party has sharpened.

"You'll see, we'll soon stuff your oligarchic rag in the cupboard," Le Pen's second-in-command, Florian Philippot said of the EU flag in a message on Twitter.

He was reacting to the European Commission's representative office in France, which posted on Twitter: "Proud of our flag, symbol of unity, solidarity and harmony between the peoples of Europe. Let's not hide it."

The EU executive was referring to Le Pen's demand on Tuesday night that French TV station remove the blue, yellow-starred EU flag from the stage where she would be interviewed, leaving just the French blue-white-and-red flag.

"I want to be the president of the French Republic. I don't want to be European Commission president. I consider the EU has done a lot of harm to our country," Le Pen told TF1 when asked to explain her demand that the EU flag, a frequent feature in such situations, in the background.

Le Pen on Monday pledged to suspend all immigration with immediate effect if elected, a proposal that does not appear in her manifesto but that touches on a subject which is crucial to her core supporters. It goes further than her previous plans to curb migration.

EU-bashing is popular with grassroots FN supporters, though opinion polls show a healthy majority of French are attached to EU membership.

(Reporting by Ingrid Melander and Simon Carraud; Editing by Brian Love)