DA leader Mmusi Maimane has said that his party would take action to "re-establish South Africa's leading voice on the continent for the entrenchment of democracy and the upholding of human rights".
This voice was lost after former president Nelson Mandela left office, he said.
Maimane said in a statement he would be travelling to Lusaka, Zambia to attend the treason trial of Hakainde Hichilema, the leader of the United Party for National Development (UPND), Zambia's largest opposition party.
Hichilema was detained six weeks ago for driving too slowly in front of Zambian President Edgar Lungu's motorcade en route to a ceremony in the Western province of the country.
He was initially charged with attempting to block Lungu's motorcade, but it was later amended to treason when it was alleged that Hichilema and 60 others had conspired to declare the opposition leader the president of Zambia.
Those charged with treason in Zambia are not allowed bail and the maximum sentence is the death penalty.
Lungu was elected president in August 2016 in a close race against Hichilema.
Maimane has slammed the South African government's "deafening silence" on the matter, which he described as "an affront to democracy across the region".
He said Hichilema's arrest points "clearly to a political motivation".
He said the silence of Zuma and the ANC said much "about their disregard for democratic values and principles on the continent".
Mailane also slammed the silence of bodies like the Southern African Development Community (SADC) and the African Union, and said he would call on other opposition leaders on the continent, including Zimbabwe's Morgan Tsvangirai and Tendai Biti, to club together "to establish a plan of action on the way forward in entrenching democratic values and the rule of law in Africa".
He added: "We must not cease in our efforts to liberate Africa from the stranglehold of 'big man politics'."
Opposition parties had a role to play in this by co-operating, he said.
Various opposition parties in the region formed the Southern African Partnership for Democratic Change forum a year ago, and Maimane is chairing it.
The aim of the forum is to stand up against the perceived abuses of governing parties in the region.
Maimane said in his statement that the DA would not be silent. "We will stand up for democracy and the rule of law on the African continent."
He called on the Zambian government to drop the treason charges and release Hichilema from prison.
"The ANC will be out of government in 2019. Authoritarian leaders in the region must know that South Africa's post-ANC government will stand up for democracy, freedom and the rule of law," he said.
Media officials for the department of international relations and co-operation were in an Africa Day event organised by the party on Thursday morning, and did not immediately respond to queries.