Washington - Republican presidential hopeful John McCain chastised his newly-declared opponent Barack Obama for his inexperience and "failed ideas," in a speech Tuesday night that signalled the start of the general election campaign.

"Tonight, we can say with confidence the primary season is over, and the general election campaign has begun," McCain told a rally in New Orleans, Louisiana, just minutes before Obama captured the Democratic presidential nomination.

McCain called Obama, who became the first African-American to get the nod of a major US political party, a "formidable" candidate that "has impressed many Americans with his eloquence and his spirited campaign."

"But I'm ready for the challenge and determined to run this race in a way that does credit to our campaign and to the proud, decent and patriotic people I ask to lead," he said.

McCain clinched the Republican Party's nomination in March, and has focussed most of his attacks on Obama over the past few weeks as it became apparent he would be the Democratic nominee.

The 71-year-old Arizona senator played on Obama's own long-running campaign theme of bringing change to Washington, telling supporters that the United States should be "concerned" by the prospect of the Democrat winning the presidency.

"The decision facing Americans in this election couldn't be more important to the future security and prosperity of American families," McCain said.

"This is, indeed, a change election," he added. "But the choice is between the right change and the wrong change - between going forward and going backward."

McCain also paid tribute to Hillary Clinton, Obama's rival for the Democratic nomination, for inspiring women around the country. The former first lady was vying to become the nation's first female president.

"The media often overlooked how compassionately she spoke to the concerns and dreams of millions of Americans, and she deserves a lot more appreciation than she sometimes received," McCain said. (dpa)