US Vice President Mike Pence walked out of Sunday's National Football League contest between the Indianapolis Colts and the San Francisco 49ers after some players took a knee during the national anthem.

The Colts were hosting the 49ers at Lucas Oil Stadium in Indianapolis, capital of Indiana, where Pence was governor prior to joining the Republican presidential ticket in 2016 as Donald Trump's running mate.

Pence said on Twitter that he and wife left the game "because @POTUS (an acronym for President of the United States) and I will not dignify any event that disrespects our soldiers, our Flag, or our National Anthem."

Though the practice of kneeling during the anthem to protest police killings of African Americans began more than a year ago, when then-49ers quarterback Colin Kaepernick took a knee, it was only a few weeks ago that Trump went on the attack against NFL players for protesting in that way.

In a speech last month in Alabama, Trump denounced the protesting players as "sons of bitches" and said that NFL owners should fire any player who kneeled during the anthem.

Trump was quick to praise Mike and Karen Pence for walking out of the Colts-49ers game.

 "I asked @VP Pence to leave stadium if any players kneeled, disrespecting our country. I am proud of him and @SecondLady Karen," the president tweeted.

 Journalists accompanying the vice president were not allowed to enter Lucas Oil Stadium.

 A member of the vice president's staff told the reporters that "there may be an early departure from the game."

 Reports of the staffer's comment to journalists led some to conclude that Pence had planned all along to walk out.

 "Wait. This was orchestrated to make a point? That's not an inexpensive thing to do," Hawaii Democratic Sen. Brian Schatz said on Twitter.