Soon after taking office in March 1933, Roosevelt declared a nationwide bank moratorium in order to prevent a run on the banks by consumers lacking confidence in the economy. He also forbade banks to pay out gold or to export it... .

On April 5, 1933, Roosevelt ordered all gold coins and gold certificates in denominations of more than $100 turned in for other money. It required all persons to deliver all gold coin, gold bullion and gold certificates owned by them to the Federal Reserve by May 1 for the set price of $20.67 per ounce. By May 10, the government had taken in $300 million of gold coin and $470 million of gold certificates. Two months later, a joint resolution of Congress abrogated the gold clauses in many public and private obligations that required the debtor to repay the creditor in gold dollars of the same weight and fineness as those borrowed.
Here are more of the specifics from Build Freedom:

In 1933 President Franklin D. Roosevelt arbitrarily reduced the amount of gold the dollar represented from one twentieth of an ounce to one thirty-fifth of an ounce. This amounted to a 43% devaluation (or debasement) of the dollar - a declaration of bankruptcy stating that the U.S. Treasury would "settle" its debts by paying 57 cents in the dollar. It also meant, in terms of gold, that overnight the government had robbed the American people of 43% of their savings. By the standards of our Founding Fathers this was a crime punishable by death.
In 1934 Roosevelt's "New Deal" took away the American citizen's right to own gold, a right Americans had enjoyed since the first pilgrims arrived. This also meant the discontinuation of [Federal Reserve] gold certificates.
In 1963 [Federal Reserve]silver certificates were discontinued. On November 26, 1963, the day of John F. Kennedy's funeral, the first 50 million "no-promise" Federal Reserve Notes were released into circulation.

President Nixon broke even more of the connection between gold and the dollar on August 15, 1971, by preventing global central bankers from redeeming dollars for gold. Notice in the original address, President Nixon says this move is "temporary":