Prime minister Boris Johnson was greeted with boos from protesters as he arrived in Cardiff Bay to meet with First Minister Mark Drakeford.
He was in Cardiff after visiting Brecon and Radnorshire ahead of the by-election and a farm in Newport.
It is the first time he has visited Wales since becoming prime minister.
First Minister Mark Drakeford said after the meeting that the prime minister had shown a "deeply concerning lack of detail" with regards to how the agricultural and manufacturing sectors would react to a no-deal Brexit.
What he has to do is back up what he says with real serious detail and evidence and that was what was missing today. I described it earlier as a vacuous optimism. There is no point in just being optimistic about the future if you have nothing to base that optimism on.
Number Ten said Mr Johnson and the first minister had discussed the "importance of the Union" during their meeting.
The PM set out how the UK will be leaving the EU on October 31st, come what may, and said he would seek to work with the Welsh Government and other Devolved Administrations, to make sure communities across the UK are ready to maximise on the opportunities that Brexit will bring.
Earlier in the day, journalists working in the Welsh media were refused the opportunity to record or film interviews with Mr Johnson.
Boris Johnson was visiting Shervington Farm where he was expected to outline his strategy for supporting farmers and workers post-Brexit.
As part of the agreement, there was a "pool" arrangement whereby an ITV network camera was able film the visit and a network journalist would ask questions to be shared by everyone.
Journalists working for Welsh media were invited to ask questions as part of a "huddle" but were later told it was not to be filmed.
ITV Wales' Political Editor Adrian Masters was at the farm covering the event and decided not to take part on the basis that any responses from the prime minister could not be recorded and therefore not shown on any broadcast outlets.
"I do think it’s a strange way to begin for a new Prime Minister who says he wants to strengthen the union to treat the main national news outlets this way", Adrian said on Twitter.
Journalist Daniel Bevan working for Heart Wales and LBC was reportedly told the same thing as well as BBC Wales News.
Our cameras did manage to record the prime minister arriving at the farm earlier today.
Photographs taken by the Press Association show Boris Johnson inspecting some chickens during his visit at Shervington Farm.
Mr Johnson is due to meet the First Minister later today. Ahead of that meeting, Mark Drakeford AM said he would be telling the prime minister a no deal Brexit would be "catastrophic for Wales".