Mugabe was preparing to take his re-election fight to urban areas amid growing international condemnation of his government's pressure on Movement for Democratic Change chief Morgan Tsvangirai ahead of presidential polls set for March 9-10.
The United States and Britain accused Mugabe of trying to fix the election after Tsvangirai, who presents the greatest threat to Mugabe's 22 years in power, was charged with treason on Monday in connection with the alleged assassination plot.
Police did not try to detain him and Tsvangirai, who denies the charge, said he did not think the government would try to stop him from contesting the election.
MDC spokesman Percy Makombe said on Tuesday police had asked the party's secretary-general, Welshman Ncube, and member of parliament Renson Gasela to report to the Criminal Investigations Department.
"They will be going to the CID as asked," Makombe said.
The official Herald newspaper reported that Mugabe was expected to hold rallies in two Harare townships on Tuesday.
The head of a 14-nation Southern African Development Community parliamentary observer group, Duke Lefhoko, said he was worried about the safety of observers and wanted talks with Mugabe's government to encourage the police to protect them.
Election observers have been stoned by Mugabe supporters and caught in an attack on opposition party offices in recent days.
"Perhaps if they gave us security personnel in plain clothes and guarantee that, especially when we attend rallies, we have the attention of the police, a police presence to see us in and out. That will assist," Lefhoko told South African radio.
Former colonial ruler Britain said the treason charge against Tsvangirai was a further sign that Mugabe, 78, was trying to rig the election.
Australia said it could impose sanctions on Zimbabwe if Tsvangirai was not allowed to take part in the poll.
New Zealand Foreign Affairs Minister Phil Goff also had harsh words for the Zimbabwean leadership.
"One would have to make the assumption that that was a jacked-up charge by the Mugabe government," he told Radio New Zealand. "This is a government that is absolutely desperate to retain power."
U.S. State Department spokesman Richard Boucher said the charge fell "against a backdrop of a well-documented campaign of violence and intimidation against the opposition" and there appeared to be no convincing evidence behind it.
An Australian television channel broadcast video footage on Feb. 13 purporting to show Tsvangirai discussing the possibility of "eliminating" Mugabe at a meeting in Montreal, Canada, last December.
Tsvangirai has denied the allegation and said his campaign would not be affected. "I am going to continue with the campaign as usual. This will not affect our political determination."
Solomon Nkiwane, a political science professor at the University of Zimbabwe, said the treason charge appeared to be part of a broader strategy.
"I think what Mugabe is trying to do here is to dramatize the charges against Tsvangirai, to torture him psychologically with this charge while it hangs around him," he said.
But analysts said the allegation could increase pressure on Mugabe by exposing him to even greater international scrutiny.
"It's doubled-edged because the international community will see this as political persecution and...will be forced to increase pressure that it has been putting on the Zimbabwe government," a diplomat in Harare said.
Canada edged away from the idea of immediate Commonwealth sanctions against Zimbabwe on Monday, but warned it could envisage Zimbabwe's eventual suspension from the group of 54 mainly British former colonies if the elections were not fair.
Tension in the southern Africa country was rising rapidly.
Last week, independent elections observers were caught up in an attack on an office of Tsvangirai's opposition Movement for Democratic Change, police shot at Tsvangirai's campaign convoy, and self-styled liberation war veterans forced a white farmer and his family to flee their property.
By Cris Chinaka