Thousands of people from different parts of the United States gathered at an Alabama bridge on Sunday to mark a bloody confrontation 50 years ago between police and protesters that forced the US government to bring about the 1965 Voting Rights Act.

Just 24 hours ago, President Barack Obama had walked atop the Edmund Pettus Bridge, police said at least 15,000 to 20,000 people had joined the crush on and around the small bridge.

Some people were seen singing hymns and others held signs, such as "Black lives matter, all lives matter."

It was on this day on March 7, 1965, when peaceful protesters were beaten and tear-gassed by police at the foot of the bridge in Selma. The incident left the whole US in a shock. The incident, however, played an important role in building momentum for passage of the Voting Rights Act later that year.

Attorney General Eric Holder commemorated the day by delivering a speech at Brown Chapel AME Church Sunday and he used the opportunity to draw parallels between the events of 1965 and today. He noted that the "Bloody Sunday" march was sparked by the murder of activist Jimmie Lee Jackson, "an unarmed, young black man."

Earlier Sunday, the officials of Selma paid tribute to the late President Lyndon Johnson for the Voting Rights Act. The attack on demonstrators preceded a Selma-to-Montgomery march, which started two weeks later in 1965. Both helped build momentum for congressional approval of the Voting Rights Act later that year.

An anniversary march from Selma to Montgomery is scheduled for today. The rally will culminate at the Alabama Capitol on Friday afternoon.