In his first speech since Democrat Joe Biden became president, he also hinted that he might run for office again in 2024.
Mr Trump strongly criticised his successor, saying US policy had gone from "America first to America last".
The speech comes weeks after Mr Trump was acquitted in an impeachment trial.
His appearance at the Conservative Political Action Conference (CPAC) in Orlando on Sunday represents his continued influence over the Republican Party.
The mood of the conference - which began on Thursday - has been extremely pro-Trump, with loyalists including Texas Senator Ted Cruz and his son Donald Trump Jr among the speakers.
The former president remains banned from social media platforms, including Facebook and Twitter, over his response to January's deadly riot at the US Capitol.
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