Fitch's decision follows four days after S&P Global downgraded South Africa's credit rating to junk status, and was widely expected.
It also said the unsettled political situation is likely to weaken "standards of governance and public finances".
The leader of the main opposition Democratic Aliiance (DA) Mmusi Maimane, who led the demonstration in Johannesburg, said the protests were about protecting the hard-earned democracy of Nelson Mandela.
It is true that numerous marchers present on 7 April called this their first march, but in my opinion, the reason for marching was greater than race today.
Protesters of President Jacob Zuma gather across Johannesburg, the economic capital of South Africa, April 7, 2017.
His actions put further strain on South Africa's economy, relegating the country's credit to "junk status".
One close Zuma ally, Water and Sanitation Minister Nomvula Mokonyane, told a WhatsApp chat group: "It's actually better Western investors will pull back and we have an opportunity to bring them back in our own terms, after we have consolidated our relations with Africa and Brics".
In Johannesburg, eNCA television showed footage of police firing stun grenades near the home of the Gupta family who are friends of Zuma and in business with his son.
Wild look to tie up series in Game 2 against Blues
But when you are only giving up two goals in each game played, it's hard to fault the goaltender for the team's struggles. Still, the Wild know what's at stake on Friday. if you let an underdog stick around for too long, they start to believe.
South Africa First Forum's Naeem Frances said residents may have to come out again in their numbers for the same cause.
The largely peaceful protests in Pretoria, Johannesburg, Durban and Cape Town appeared to attract a diverse mix of South Africans, but Zuma railed against those who participated.
MK veterans‚ wearing camouflage uniforms‚ were lined up outside Luthuli House to "protect" it from protesters calling on President Jacob Zuma to quit.
Al Jazeera's Fahmida Miller, reporting from Pretoria, said the demonstrations are significant because "it's just the beginning of protests and marches by a large sector of South Africans calling for the president to resign".
"We are all gathered here today with one call that President Zuma must step down today because he is bad for our economy and that kills chances of employment", said Filtane. On 18 April 2017thousands of people will gather outside Parliament when MP's debate a vote of No Confidence in Zuma, the people will not get exhausted until Zuma is no more in that office.
Zuma's ruling party, the African National Congress, has an absolute majority in parliament, but the opposition expects numerous ruling MPs to break the party discipline and vote conscientiously.
Earlier this week, a major decision-making body within the ruling ANC party discarded a complaint against Mr Zuma that he had failed to consult executives before reshuffling his cabinet.
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