Riot police, mounted officers, a force helicopter and dog units were being used in Glasgow after protesters against an Irish unity march sparked "significant disorder".
Police said the planned march through the city's Govan area, organised by the James Connolly Republican Flute Band, was met by hundreds of "disruptive" counter demonstrators at around 7pm.
The force said this led to "significant disorder" around Govan Road, which was blocked by officers.
Chief Superintendent Mark Hargreaves said: "Police Scotland has a duty to facilitate processions and any peaceful protest, but this kind of behaviour by persons demonstrating against the parade is utterly unacceptable.
"It is extremely disappointing to see people acting in this fashion, causing fear and alarm to members of the public as well as putting many people at risk.
"A range of policing resources are in place as part of a multi-agency response.
"Specialist public order officers in full protective equipment, the mounted section, air support and dog units have been deployed, and we are using proportionate tactics to de-escalate the situation as quickly and safely as possible.
"Police Scotland will undertake a thorough and robust enquiry, and take any necessary action against those found to have been causing disruption."
Witnesses reported smoke bombs being used.
Glasgow City Council advised of the road block in a traffic bulletin and Govan Subway Station also closed due to the incident but has since reopened.
Glasgow City Council said: "Due to an ongoing Police incident Govan Road is blocked between Golspie Street and Elder Street.
"Drivers are advised to avoid the area if possible."
Reporting by PA.
