Trump is expected to meet Russian President Vladimir Putin in Slovenia on Friday.
"While we will always welcome new citizens who share our values and love our people, our borders will always be closed to terrorism and extremism of any kind", he added to applause from the 5,000-strong crowd.
Speaking to reporters ahead of Trump's arrival, Jaroslaw Kaczynski, who leads the ruling Law and Justice party (PiS), said others in Europe are envious of the visit, mentioning Britain in particular.
Energy security, the Three Seas Initiative and NATO's role were major issues raised during U.S. President Donald Trump's visit to Poland on Thursday. Activists from a small left-wing party also held a protest, during which they protested Trump's apparent suppression of women's rights by dressing up as characters from The Handmaid's Tale.
"The story of Poland is the story of a people who have never lost hope, who have never been broken, and who have never forgotten who they are", he will say. The New York Times also reported that numerous attendees had been bussed in from the more conservative countryside, as it was feared Trump would not draw a large crowd while speaking in the more liberal city of Warsaw.
Mass rally held ahead of G20 summit in Hamburg
The highly anticipated meeting with [sic] also be the first time Trump comes face to face with the Russian leader since his election.
During a press conference in Poland with President Andrzej Duda Thursday, President Donald Trump jumped at another opportunity to slam the US media, specifically CNN, saying the news outlet took a video Trump tweeted over the weekend "too seriously".
Trump is next headed for Hamburg, Germany, where he will attend the G20 summit.
"We've reiterated our enduring bond", Trump said.
On the matter of Russia's interference in the 2016 US election, Trump suggested he was not yet convinced Moscow was exclusively responsible.
Poland is one of the largest buyers of Russia's natural gas.
North Korea test-fired its first intercontinental ballistic missile Tuesday, marking a significant step forward in its weapons program and an escalation of a nuclear standoff with the U.S.