Chaos erupted in eSwatini as police fired teargas and water canons at thousands of civil servants protesting against low wages on Wednesday night.
Teachers and workers accused King Mswati III of not providing sufficient wages and lamented over the rising cost of living.
More than 3000 people took to the streets in the kingdom's capital Mbabane on Wednesday.
"Our problem is that we have a selfish king," Ngwane National Liberatory Congress pro-democracy group chief Sibongile Mazibuko told AFP. "He loots national coffers to satisfy his personal greed."
The Congress of SA Trade Unions in Mpumalanga threw its weight behind the protest action and vowed to block Mahamba and Matsamo border posts in solidarity with the eSwatini public servants.
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"The border blockades are convened in solidarity with the eSwatini public sector unions who are currently fighting for a cost of living adjustment," Sizwe Pamla, a spokesperson for Cosatu said.
"Mpumalanga Cosatu supports the strike by eSwatini's National Public Services & Allied Workers Union [NAPSAWU] and other public sector unions of Swaziland and holds a strong view that their demand for a living wage is a genuine one which must be acceded to by the government of Swaziland. Cosatu will be leading border protests alongside its national affiliate NEHAWU."
Government workers in eSwatini are fighting for a 7.8 percent salary increase.