By Ian Simpson and Jeff Mason
WASHINGTON (Reuters) - The United States is struggling to realize the vision that civil rights leader Martin Luther King described in his famous "I Have a Dream" speech 50 years ago, President Barack Obama said on Wednesday, citing economic security as a still elusive goal.
Obama, the first black U.S. president, spoke to thousands of marchers on Washington's National Mall to commemorate King's landmark address, which came to symbolize the struggle for equality among blacks and whites in America.
Joined by members of the King family and two former presidents, Obama and his fellow speakers urged Americans to continue the slain leader's quest for justice.
Obama said King's speech inspired millions of Americans to fight for a more equal society and rights that people now take for granted.
"To dismiss the magnitude of this progress, to suggest, as some sometimes do, that little has changed, that dishonors the courage, the sacrifice of those who paid the price to march in those years," Obama said.
"But we would dishonor those heroes as well to suggest that the work of this nation is somehow complete," he said, calling economic justice the "unfinished business" of the civil rights battle.
Marchers, many wearing T-shirts with King's face on them, began their walk near the U.S. Capitol.
They were led by a line of military veterans and people who had been at the 1963 march, their arms linked. People sang "We Shall Overcome" and other civil rights anthems.
Fighting restrictive voting rights laws that Democrats say hurt minorities, combating joblessness and reducing gun violence among African Americans are among the issues that civil rights leaders have put at the forefront of their efforts in 2013.
"This march was supposed to be about jobs, but it's about a lot more," said marcher Ash Mobley, 27, of Washington, who said she was there to represent her grandmother, who had been at the 1963 event.
The marchers heard speeches from former presidents Bill Clinton and Jimmy Carter and members of King's family on the steps of the Lincoln Memorial, the site of King's address on August 28, 1963.
A bell rang at 3 p.m. EDT (1900 GMT), 50 years to the minute after King ended his clarion call of the civil rights movement with the words "Let freedom ring."
King's speech is credited with helping spur passage of sweeping civil rights laws. A white prison escapee assassinated him in 1968.
King's elderly sister, Christine King Farris, said her brother's dream was alive, if unfulfilled.
"Yes they can slay the dreamer, but no, they cannot destroy his immortal dream," she said.
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Bernice King, the slain leader's youngest child, urged the crowd to stay true to the ideals enunciated by her father.
"If freedom is going to ring in Libya, in Syria, in Egypt, in Florida, then we must reach across the table, feed each other and let freedom ring," she said.
Obama's address commemorating King, a Nobel Peace Prize winner and advocate of non-violence, came as the White House edges closer to launching military strikes in Syria in response to what U.S. officials say they believe was a chemical weapons attack by the Syrian government on civilians.
Obama made no mention of Syria in his address.
The president, whose mother was white and whose father was black, has sometimes seemed reluctant to weigh in on persistent racial divides in the United States, but he spoke forcefully about the issue last month after the man who killed black Florida teenager Trayvon Martin was acquitted.
On Wednesday Obama said the 1960s civil rights movement led to greater freedoms for many different groups of U.S. society.
"Because they marched, city councils changed and state legislatures changed, and Congress changed, and, yes, eventually, the White House changed," he said to applause, a nod at his own historic election.
"Because they marched, America became more free and more fair - not just for African Americans, but for women and Latinos, Asians and Native Americans, for Catholics, Jews, and Muslims, for gays, for Americans with a disability," he said.
The "Let Freedom Ring and Call to Action" ceremony came as almost half of Americans say much more needs to be done before the color-blind society King envisioned is realized.
Wednesday's event capped a week-long celebration of King's historic call for racial and economic justice. They included a march on Saturday that drew thousands of people urging action on jobs, voting rights and gun violence.
Obama and Clinton also used their remarks to chide lawmakers in Washington for partisan battles that have prevented agreement on issues such as the budget and immigration reform.
"I would respectfully suggest that Martin Luther King did not live and die to hear his heirs whine about political gridlock," Clinton said. "It is time to stop complaining and put our shoulders against the stubborn gates holding the American people back."
(Additional reporting by Mark Felsenthal, Roberta Rampton and Steve Holland; Editing by Karey Van Hall and Lisa Shumaker)