In Mandalay, Myanmar's second largest city, security forces arrested dozens of university students who staged a street protest on Sunday, a witness said.
The junta has not commented on Gambari's mission. Since arriving on Saturday, he has met with junior members of the junta in Naypyitaw and pro-democracy leader Aung San Suu Kyi in Yangon on Sunday.
Gambari's hour-long talk with Aung San Suu Kyi was unexpected -- he did not know before he arrived if he would be allowed to meet the 1991 Nobel Peace prize winner, who has come to symbolize the struggle for democracy in Myanmar. She has spent nearly 12 of the last 18 years under house arrest.
Aung San Suu Kyi's National League for Democracy party was not optimistic Gambari would wield any influence over the junta leaders.
The junta has never responded well to international pressure in the past and has rebuffed UN efforts to bring about reconciliation with Aung San Suu Kyi.
But its desire for oil and gas investment, increased tourism and its status as a member of ASEAN means it cannot follow a completely isolated path, as it has in the past.
"I do think a number of underlying dynamics have changed quite fundamentally and make us more hopeful that something might happen," British Ambassador to Myanmar Mark Canning said.
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