Workers fear their safety is being put at risk by being told to go to work during the coronavirus pandemic.
The UK government has said people should only travel to work if it is absolutely essential or they are key workers.
One mechanic said he had been told he could lose his job if he stays at home.
The body representing trade unions in Wales said current guidance was vague and open to abuse.
New measures, which are in place for at least three weeks, state people across the UK should only leave home to go to work "where this is absolutely necessary and cannot be done from home".
Some businesses in Wales, including the Sony plant in Bridgend which makes medical equipment, have been classed as essential by the UK government.
But workers from various industries said they were being made to work in crowded offices and warehouses, with little to no protection from contact with others.
TUC Cymru, which represents thousands of workers across Wales, said it had been contacted by a large number of members who were being told they had to go into work.
Policy officer Nisreen Mansour said some fashion workers were still being made to go to work on Monday, with very little protection from the virus.
"The government advice is not firm enough and does not give people the right to refuse to go to work," she said.
"Eventually if they were to be let go because of the decision to self-isolate, they could then take them to tribunal, there would be grounds there to say you were dismissed unfairly."
'Someone was selling hand sanitiser on the sorting office floor'
A south Wales postal worker said he feared staff were being put at risk to deliver non-essential post, including takeaway leaflets and flyers.
The man works in a sorting office and said there was no hand sanitiser, a lack of gloves to handle parcels, and no way to keep away from colleagues.
"We understand having to sort medical supplies, testing, and food supplies, but we are still distributing takeaway leaflets and junk mail and I don't understand it, we are putting ourselves and the public at risk for this," he said.
He said someone had been selling hand sanitiser to staff and managers were still having huddles in close contact with each other.
"I feel concerned, personally I do not feel safe, at the end of the day I am really worried about passing it on," he said.
The Royal Mail said it took the health and safety of workers and customers very seriously and had introduced a range of new social distancing measures aimed at offering further protection for staff.
'They are bullying us into going to work'
A mechanic whose wife is in the high risk category for contracting the virus said he was being threatened with disciplinary action if he did not turn up for work.
The man, who works at a garage in south Wales, said staff had been told if they did not come in, they would be recorded as "going Awol" , despite having no customers.
"We are trying to keep so many metres away from people, but it is difficult when you are working on the same job," he said.
"We do not know if people's cars are contaminated, we do not know if people we are working with have got it.
"It is the people higher up they are all sat at home, they are sending out emails forcing us into work. I do not understand why they are risking everyone's health."
'We sat at desks next to each other'
Working with hundreds of other people in an office building, one finance worker said people were still hot desking and not socially distancing.
The man, who works on Payment Protection Insurance refunds, said while senior staff were working from home, staff who were not high risk were still having to travel to work.
"We have been told that we are key workers because the company works in the financial sector, but I do not think this is essential," he said.
"I moved desks twice this morning to try and keep myself in a safe situation, because of my concerns I have come home on unpaid leave until home working is arranged or until this is over."
Call centre workers 'unsung heroes'
A body representing call centre workers said while sales workers should not be going into work, many contact workers were a lifeline for people at home.
Sandra Busby, of Wales Contact Centre Forum, said most contact centres were deep cleaning their offices between shifts and working on reduced staff.
But she said people needed to work in the centres to help people contact gas, electricity and water companies and financial services while people were in isolation.