JOHANNESBURG, JULY 26 -- President Frederik W. de Klerk met tonight with African National Congress leader Nelson Mandela to head off a looming crisis over government allegations of a Communist-led plot within the ANC to stage an armed insurrection if negotiations with the government fail.

The arrest of about a dozen Communist activists allegedly involved in the plot, including one senior ANC official, appeared today to have jeopardized a crucial meeting between the government and the ANC set for Aug. 6.

That meeting is expected to resolve the final obstacles to the start of formal negotiations between South Africa's white and black leaders for a new constitution ending apartheid and establishing a non-racial democracy.

A statement issued from de Klerk's office said he and Mandela had met tonight and that they agreed to meet again next week at Mandela's request.

In a statement earlier today, de Klerk charged that the ANC did "not appreciate the seriousness of the facts which were brought to light by the South African police investigation of the past few days."

Wednesday night, police arrested a leading member of the South African Communist Party and the ANC in connection with the alleged armed insurrection plot within the two allied organizations.

The arrest of Mac Maharaj, a member of the ANC National Executive Committee and the Communist Party Central Committee, provoked speculation that the ANC might retaliate by postponing its next round of talks with the government.

However, the ANC remained silent about Maharaj's arrest, apparently waiting to see whether other members of its National Executive Committee would be taken into custody.

Maharaj had been in exile for 13 years when he was granted temporary immunity by the government and allowed to return to South Africa in May. A Justice Ministry spokeswoman said his immunity only covered crimes committed before May 19.

Communist Party spokesman Jeremy Cronin denounced Maharaj's arrest as a "cynical and blatant attempt" by the police to disrupt a Communist Party rally scheduled for Sunday, the first since the party was legalized Feb. 2.

Maharaj is the chief organizer for the rally, which is to take place in the black township of Soweto.

The police reportedly have arrested about a dozen ANC activists in connection with the alleged insurrection plot, most of them members of both the ANC's military wing, Spear of the Nation, and the Communist Party.

But ANC officials say the police have arrested as many as 150 activists since the ban was lifted on the Communist Party and, under Article 29 of the Internal Security Act, continue to hold them without access to lawyers or family members.

In homes belonging to alleged ANC and Communist Party members, police reportedly have discovered arms caches as well as computer files containing 4,000 pages of ANC records and materials that have provided the basis for the government's charges against the African nationalists.

Both ANC and Communist Party officials say they believe the government is mounting a campaign to drive a permanent wedge between the closely allied Communists and the African nationalists.

Wednesday, Mandela denounced the allegation of a Communist plot as "totally unfounded" and said he knew of no one in his organization who was not committed to the process of peaceful negotiations with the government.

The African nationalists also seemed to defy any government efforts to split their alliance with the Communists by naming a delegation for the talks that includes the Communist Party's general secretary, Joe Slovo, and the ANC's military commander-in-chief, Joe Modise, also believed to be a Communist Party member.

Meanwhile, another senior ANC and Communist Party official, Ronnie Kasrils, said there had never been any plan to organize an uprising against the South African government.

"I can assure all of you that there is no blueprint; there's been no plan to organize an insurrection at a particular time," said Kasrils, who was head of the ANC's military intelligence unit until 1988.

"What the gentlemen of the Special Branch {South African police} are doing and saying in their pristine naivete is based on revolutionary discourse and nothing else," Kasrils said.

Kasrils said the police almost had arrested him along with Maharaj Wednesday night, but that he had escaped.

Kasrils said he thought the police wanted them both because of their extensive involvement in setting up the ANC's underground operation.