Former US president Donald Trump on Saturday welcomed his acquittal in the US Senate on an impeachment charge and said his political movement "has only just begun."

"Our historic, patriotic and beautiful movement to Make America Great Again has only just begun," Trump said in a statement issued just moments after the Senate vote.

"In the months ahead I have much to share with you, and I look forward to continuing our incredible journey together to achieve American greatness for all of our people," he said.

"We have so much work ahead of us, and soon we will emerge with a vision for a bright, radiant, and limitless American future," Trump said.

A two-thirds majority was needed in the 100-member Senate to convict Trump of the charge of inciting the January 6 attack on the US Capitol by his supporters but only 57 senators voted "guilty."

Seven Republicans joined Democrats in the Senate in voting to convict the former president.

Trump denounced his second impeachment trial as "yet another phase of the greatest witch hunt in the history of our Country."

"No president has ever gone through anything like it," Trump said.