"Not one American flag on the massive stage at the Democratic National Convention until people started complaining-then a small one. Pathetic," Trump tweeted.
He also was critical of the mainstream American media for praising Clinton despite her not holding a news conference.
"It's been 235 days since Crooked Hillary Clinton has had a press conference and you as reporters who give her all these glowing reports should ask yourselves why. I'll tell you why: despite the nice platitudes, she's been a mess," Trump said.
The 70-year Republican faces Clinton in a do or die battle in November.
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In another post on his Facebook page, Trump alleged for months, the Democratic National Committee (DNC) has scoffed while both him and Senator Bernie Sanders relentlessly contended that the primaries were rigged in favor of Clinton.
But now there is evidence to prove that they were right, evidence that calls the legitimacy of the entire Democratic Party primary process into question, he alleged.
Trump slammed the Democratic Party leadership for not mentioning ISIS in their speeches.
"Hopefully the violent and vicious killing by ISIS of a beloved French priest is causing people to start thinking rationally. Get tough!" he tweeted.
"Crooked Hillary Clinton wants to flood our country with Syrian immigrants that we know little or nothing about. The danger is massive. NO!" he alleged.