Mark Drakeford has slammed the Tories as "one of the most awful Governments we have ever seen".
The First Minister of Wales praised Labour for achieving so much across the four nations despite being "at the teeth" of Boris Johnson's cabinet.
But he called on his peers to turn "thought into practical action" on the road to power in Westminster.
Speaking after a raucous applause, Mr Drakeford said: "Everything Labour has achieved in Wales, across England and in Scotland, has been achieved in the teeth of one of the most awful UK Governments we have ever seen.
"Incompetent to its core and automatically hostile to anyone who does not share its visceral reactionary instincts at home or abroad."
In a sly dig at Sir Keir Starmer's indecisiveness on devolution, Mr Drakeford added: "Just imagine what we could do if we had a UK Labour Government committed to renewing and rebuilding the United Kingdom so it genuinely worked for everyone.
"One that recognises that devolution is the UK’s greatest strength, not its greatest mistake."
Earlier today, Mayor of Greater Manchester Andy Burnham said Labour needs to become "doers and not sayers".
Speaking at a Labour Conference fringe event, Mr Burnham said: "Buy into devolution and you can start doing deeds and not words. And it makes politics more meaningful for people."
Mr Drakeford's uplifting comments about the Labour party would raise hopes within party HQ, following days of reported infighting and swipes at leader Sir Keir Starmer.
