"I am concerned about the escalation of violence in Bangkok in recent days and express regret over the loss of lives and injuries," Ban told reporters on the sidelines of a UN Industrial Development Organization (UNIDO) conference in Lima.
Ban also expressed concern over attempts by demonstrators to "forcefully seize government buildings and media outlets."
"I once more call for all sides to exercise utmost restraint and to resolve political differences through dialogue, through peaceful means," Ban said.
"All parties need to observe full respect for democratic principles, the rule of law, and human rights, including freedom of expression."
Thailand has been gripped by the worst political violence in the country for three years, with demonstrators demanding the resignation of Thai Prime Minister Yingluck Shinawatra.
Police used rubber bullets, tear gas and water cannon against rock-throwing demonstrators as they strengthened their defence of key government buildings, after weekend unrest in the capital left several dead and more than a hundred wounded.
The protests, aimed at unseating the elected government and replacing it with a "people's council," are the latest outbreak of civil strife to rock the kingdom since royalist generals ousted Thaksin Shinawatra, Yingluck's brother, seven years ago.