Media playback is unsupported on your device Media caption "I think it's terrible...I think it's a disgrace", Trump said of the controversial memo

The US Congress has released a memo accusing the FBI of abusing its power in its investigations into Donald Trump's presidential campaign.

The memo, written by Republicans, alleges the FBI used unsubstantiated evidence to spy on a Trump aide.

The FBI had warned against the memo's release and said key facts had been omitted.

Democrats said it was aimed at derailing ongoing investigations into the Trump campaign's links with Russia.

The memo has become a flashpoint in the bitter dispute between Republicans and Democrats over investigations into whether Russia colluded with the Trump campaign in the 2016 election.

What's in the memo?

It centres on court-approved wiretapping of Carter Page, a foreign policy adviser to the Trump campaign who was put under electronic surveillance by the FBI.

But the memo accuses the FBI and the justice department of using an unsubstantiated and Democratic-funded report to obtain the October 2016 warrant that gave permission to spy on Mr Page.

It says that they did not tell the authorities their claim to the warrant was partially based on a dossier funded in part by the rival Hillary Clinton campaign.

It also says that the author of that dossier, a former British intelligence agent called Christopher Steele, told a senior justice department official that he was "desperate" that Donald Trump not win the vote.

The report says that all this represents "a troubling breakdown of legal processes established to protect the American people from abuses".

The memo was top secret, but it was approved for release by the House Intelligence Committee on Monday and by Mr Trump on Friday.

Analysis: Bomb or dud?

By the BBC's North America reporter Anthony Zurcher

The mystery is over, the memo is out, and the results are… pretty much what everyone expected.

Whether the Republican-generated document is as explosive as it had been made out to be depends on how one views the now-infamous Christopher Steele dossier and whether one believes the memo's assertion that it was an "essential part" of the Carter Page Fisa warrant's approval - or if there was other pertinent information the Republican memo-writers omitted.

The memo makes the case that the Fisa judge should have been told about information about Steele that could call his objectivity into question - including his expressed views about Donald Trump, his contacts with the press and the fact that his investigation was funded, in part, by Democratic Party interests.

Would such a disclosure have been enough to make the Page warrant request one of only a handful of the tens of thousands of Fisa applications that have been rejected by judges since the system was set up in 1978? And is the surveillance of Page - who had drawn the attention of US intelligence services as far back as 2013 - enough to call into question the entire Russia investigation, which had been initiated months before the warrant was approved?

The answers to those questions will determine whether the memo was a bomb or a dud.

Read more from Anthony

How have Republicans reacted?

Republicans who support the release of the memo say it exposes malpractice and political bias within the FBI and justice department.

Asked about the contents of the memo, Mr Trump said a lot of people should be "ashamed of themselves".

Earlier on Friday the president accused top officials of politicising FBI and justice department investigations to damage his Republican party.

Devin Nunes, who commissioned the memo, has said it shows "serious violations" of public trust and he hopes it will trigger reforms.

Mr Page, the Trump aide who was the subject of the surveillance, said he would use the memo in upcoming legal action against the justice department.

But not all Republicans supported the release of the memo. Senator John McCain strongly criticised his party colleagues for attacking the FBI and the justice department, and accused Mr Trump of undermining the rule of law.

What has the other reaction been?

Democrats say the memo is a "shameful effort to discredit" the FBI and inquiries into Russian meddling in the 2016 election.

Democratic House leader Nancy Pelosi said: "By not protecting intelligence sources and methods, [Trump] just sent his friend Putin a bouquet."

Democrats have warned Mr Trump in a letter not to use the memo as a "pretext" to fire senior justice department officials or the special counsel appointed to investigate alleged Trump campaign ties to Russia.

They said this would provoke a constitutional crisis.

Meanwhile, FBI agents say they "have not, and will not, allow partisan politics to distract" from their work.

In an email to his staff, FBI Director Christopher Wray said: "Talk is cheap; the work you do is what will endure.

"I stand by our shared determination to do our work independently and by the book. I stand with you," he said.

And former FBI Director James Comey, who was fired by President Trump in May, tweeted that the memo was "dishonest and misleading".