WASHINGTON, Sept 14 (Reuters) - U.S. Senator Amy Klobuchar, the top Democrat on the Senate Judiciary Committee's antitrust panel, introduced a bill on Thursday that would make it easier for the government to stop mergers and another that would raise fees that companies pay for merger reviews.

Antitrust, which usually gets little public attention, has had a higher profile recently due to aggressive mergers in concentrated industries as diverse as agrochemicals, entertainment ticket sales, supermarkets and airlines.

Concerns that the mergers could lead to higher prices and less innovation in these industries has prompted some activists in the party to press harder for tougher antitrust law and more vigorous enforcement.

One bill would lessen the burden on the Justice Department and Federal Trade Commission, which review mergers to ensure they comply with antitrust law, by saying that in the case of certain mega-mergers that the companies would have to prove that the proposed deal does not hurt consumers, Klobuchar's office said in a statement.

Currently, the agencies must prove to a judge that consumers would be harmed.

For smaller deals, if the agency wanted to stop a transaction they would have to show that the proposed acquisition would "materially" lessen competition, rather than "substantially," according to the text of the bill.

"By strengthening antitrust enforcement, we can protect people from harmful mergers that threaten competition, drive up costs for consumers, and limit innovation and economic opportunity," Klobuchar, who represents Minnesota, said in a statement.

The bill is co-sponsored by Democratic Senators Richard Blumenthal of Connecticut, Ed Markey of Massachusetts and Kristen Gillibrand of New York.

Under the second bill, smaller deals, $500 million or less, would see their fees cut substantially while the biggest deals would see them go up. For deals worth $5 billion or more, the fee would go from $280,000 to $2.25 million.

This bill is co-sponsored by Blumenthal, Markey and Gillibrand as well as Democratic Senators Patrick Leahy of Vermont, Al Franken of Minnesota, Cory Booker of New Jersey, Dick Durbin of Illinois, Mazie Hirono of Hawaii and Tammy Baldwin of Wisconsin.