Republican presidential candidate Donald Trump has described as "rigged" the way the US state of Colorado picks its presidential nominee.

His rival Ted Cruz swept the board in that state, where delegates are selected by senior party activists.

Mr Trump is still well ahead in the Republican race but if he does not get 1,237 delegates in the state-by-state contests, he could lose out anyway.

A winner could well emerge from a contested convention in July.

In that scenario, many of the delegates can back who they want to after the first ballot, opening the door for Texas Senator Mr Cruz or even the third candidate in the race, John Kasich.

Senior Republicans have voiced concern about Mr Trump's chances of winning in November's general election, and oppose his hardline views on immigration.

Next week New Yorkers get the chance to pick their nominees in the Republican and Democratic contests, in a pivotal moment in the process.

But on Monday, Mr Trump, a New York businessman, told Fox News he was angry about the way Mr Cruz had won all the delegates in Colorado.

"They weren't given a vote. It's a crooked deal. The system is rigged, crooked. That's not the way democracy is supposed to work."

Delegates are party members with the power to vote for each candidate at the party conventions held in July, where the nominee is formally confirmed.

In the modern political era, most states have opted to hold state-wide primaries or caucuses to determine the number of delegates pledged to a particular candidate.

But Colorado decided to select its delegates in a different way, at its own state convention.

His new campaign manager Paul Manafort accused the Cruz campaign of "Gestapo tactics" in how they had persuaded delegates to support him.

Mr Trump told a rally on Sunday that the person who wins the most votes in the primary process should automatically be the nominee.

"What they're trying to do is subvert the movement with crooked shenanigans," he said.

He also revealed that only one of his three children residing in the state of New York will be able to vote.

"They didn't register in time," he said. "So they feel very, very guilty."

One of Mr Trump's most controversial policy pledges is to build a wall on the southern border with Mexico, and to deport 11 million undocumented migrants.

His mantra to "build a wall" appears to have spread to some school playing fields.

In Wisconsin, a schoolgirls' football match had to be stopped when the slogan became a chant directed at Hispanic players, local media reported.

The school district confirmed to the BBC that it was investigating the allegations.