Former US president Barack Obama, in his sharpest attack to date on President Donald Trump, condemned the use of federal agents against protesters and attempts at voter suppression.

"Today we witness with our own eyes, police officers kneeling on the necks of black Americans," Obama said in a eulogy at the funeral service in Atlanta of civil rights leader John Lewis.

"We can witness our federal government sending agents to use tear gas and batons against peaceful demonstrators," he said.

Obama did not mention Trump by name but he was clearly referencing recent moves by the Republican president, who used troops to clear protestors from Lafayette Square outside the White House and sent federal agents recently to Portland, Oregon.

Obama also took a jab at Republican attempts to suppress the minority vote and Trump's repeated attacks on mail-in voting.

"Even as we sit here, there are those in power who are doing their darnedest to discourage people from voting by closing polling locations and targeting minorities and students with restrictive ID laws and attacking our voting rights with surgical precision, even undermining the postal service in the run-up to an election that's going to be dependent on mail-in ballots so people don't get sick," he said.

Trump launched another broadside on Thursday against mail-in voting, which is expected to play a more prominent role in the November election because of the coronavirus outbreak.

"With Universal Mail-In Voting (not Absentee Voting, which is good), 2020 will be the most INACCURATE & FRAUDULENT Election in history," Trump said. "It will be a great embarrassment to the USA. Delay the Election until people can properly, securely and safely vote???"

Paying tribute to Lewis, who died on July 17 at the age of 80, Obama said the former Democratic congressman did "everything he could to preserve this democracy and as long as we have breath in our bodies, we have to continue his cause."