Lamp posts, street signs and cars carry the banners, which bear mottos like: "A traitor of Pakistan army is a traitor of the country" and "We love Pakistan army and ISI," referring to its Inter-Services Intelligence wing.
The mystery signs arrived in Pakistan as its powerful military faces off with the country's largest private television station over allegations that its forces were behind a shooting that seriously wounded one of its top anchors.
The controversy started last Saturday when gunmen opened fire on Hamid Mir, an anchor for Geo News, wounding him six times.
After the shooting, his journalist brother appeared on Geo and blamed the ISI for the shooting while the station showed a photo of its chief, Lt Gen Zaheerul Islam. The station repeatedly aired the accusations and blamed the ISI for the "assassination attempt" against Mir.
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The station then reported that its signal was blocked in areas of the country as small rallies supporting the military began. Last week, the posters began appearing, some with the ISI leader on them.
Their origin is a mystery. Some said they were from the people of Islamabad, the capital. Others mentioned a Pakistani religious figure. Some listed the All Traders Welfare Association, a little-known trade group in the capital, headed by a man named Furqan Murtaza.
When reached by The Associated Press, Murtaza denied that any government or military agency encouraged his campaign.
Generally, people have to get permission before hanging posters and banners in the capital and pay a fee.
An official from the Capital Development Authority, which manages Islamabad, said the agency did not receive any requests to hang the posters, though it is common for people to do it without permission.