President Donald Trump expressed disbelief Sunday that people still want him to release his tax returns and insisted someone must have paid for Saturday’s nationwide protests demanding he do so.

In a series of tweets Easter morning, Trump questioned the validity of post-election demonstrations against him and called for an investigation into who must have “paid for the small organized rallies.”

I did what was an almost an impossible thing to do for a Republican-easily won the Electoral College! Now Tax Returns are brought up again? — Donald J. Trump (@realDonaldTrump) April 16, 2017

Someone should look into who paid for the small organized rallies yesterday. The election is over! — Donald J. Trump (@realDonaldTrump) April 16, 2017

Saturday’s protests were among the biggest anti-Trump demonstrations since January’s Women’s March and totaled tens of thousands of people in around 100 cities.

Organizers estimated that over 25,000 people gathered in Washington, D.C., and another 20,000 people assembled in New York City.

Held on the date when Americans’ taxes are normally due, the protests chided Trump for being the first president in four decades not to release his tax returns or comparable financial information ― a standard established when former President Richard Nixon released his returns after he was audited.

His refusal to release them, critics say, indicates a possible cover-up of tax avoidance, financial ties to the Russian government or other monetary misdeeds.