WASHINGTON -- Google's decision to honour the birthday of U.S. labour organizer Cesar Chavez angered some American Christians on Sunday, who fumed that it was disrespectful to celebrate Chavez with a so-called Google Doodle on Easter Sunday.

The face of Chavez, a Mexican immigrant who organized Latino farm workers in the 1960s, was situated in the middle "o" of the Google logo on Sunday as the search engine giant opted against recognizing a secular holiday to commemorate what would have been the civil rights activist's 86th birthday.

Chavez, who died 20 years ago, was a co-founder of the United Farm Workers, a union dedicated to fighting for decent pay, humane working conditions and safe housing for farm workers. Two years ago, President Barack Obama declared March 31 Cesar Chavez Day.

Conservative websites assailed Google's decision.

The Daily Caller expressed confusion about why Google "chose specifically to honour Chavez's birthday, instead of Easter Sunday."

The conservative news organization also suggested Obama might have influenced Google's thinking. Google CEO Eric Schmidt was an "informal adviser" in both of Obama's presidential campaigns, the Daily Caller reported, was a member of his transition team in 2009 and is apparently rumoured for a cabinet position during the president's second term.

Conservatives took to social media to express outrage that Google would opt to honour a "socialist."

"Today I switch to Bing since #google thinks Christ is less important that Cesar Chavez, socialist labor leader," tweeted Lisa Schreckenstein.

Tweeted another: "Congrats Google, you've managed to alienate all Christians in America today: instead of celebrating Christ, they celebrate Cesar Chavez."

Cesar Chavez Day, however, has become a popular day of celebration in states with significant Hispanic populations. In some cities, the revelry rivals the type seen on Halloween and St. Patrick's Day.

In San Antonio, Texas, hundreds of people took part in the city's 17th annual Cesar Chavez march over the weekend, calling for comprehensive immigration reform.

"We're all fighting to ensure there is going to be a favourable vote taken in Congress this year and that President Obama has the opportunity to sign a new bill granting legal status to farm workers," said Arturo Rodriguez, now the president of the United Farm Workers.

Others on social media praised Google for honouring Chavez and mocked those who confused him with Hugo Chavez, the recently deceased Venezuelan president.

The Twitter account for The Twitchy, conservative pundit Michelle Malkin's right-wing news outlet, initially claimed Google was honouring the late revolutionary.

"My Easter miracle: seeing how many people confuse Cesar Chavez for Hugo Chavez. Thanks, @Google doodle!" Tweeted one.

Added LOLGOP, a Twitter account devoted to mocking Republicans and the Christian right: "I'm glad conservatives have turned Easter into a chance to cast verbal stones at Cesar Chavez. Nothing ironic about that at all."