"It's a rigged, crooked system that's designed so that the bosses can pick whoever they want and that people like me can't run and can't defend you against foreign nonsense," said the business mogul.
Later, he complained about "so much phony stuff" -- including "The Republican Party voting and the Democrats."
Trump campaigned here fresh off a dominating victory in his home state of New York on Tuesday. Indiana's May 3 primary is shaping up as a critical battleground in the rivalry between Trump and Texas Sen. Ted Cruz.
"I had to come early," Trump said at the top of his remarks.
So far, Cruz has outmaneuvered Trump in the arcane process of getting supporters selected as delegates to the national convention in Cleveland. Most of the delegates are bound to the winner of their states or congressional districts on the first ballot. But if no one gets a majority, many will be freed up to choose someone else -- posing a risk for candidates who don't help install loyal backers.
Trump mentioned Pennsylvania, which votes next week and sends most of its delegates to the convention officially uncommitted.
Trump also praised Indiana Gov. Mike Pence, whom he met with before his rally. Pence has yet to make an endorsement in the race.
Trump brought up the air-conditioner maker Carrier several times in his remarks, a company that planned to move jobs from Indianapolis to Mexico. Trump has highlighted Carrier in the past, saying that he would impose a tax penalty as president for such a move.
The front-runner was interrupted by protesters throughout his rally. Before his rally, an announcement was played encouraging attendees not to "touch or harm" protesters but to point them out so officers could remove them.
After referring to Cruz more cordially as Senator Cruz on Tuesday night, he returned to his go-to negative label: "Lyin' Ted."
And for Democratic presidential front-runner Hillary Clinton, Trump reserved "Crooked Hillary."