Serhatcan Yurdam, via Instagram

As my colleagues Tim Arango, Sebnem Arsu and Anne Barnard report, the conflict in Syria spilled over its border with Turkey again on Thursday, as the Turkish military struck Syrian positions in retaliation for cross-border shelling that killed five Turkish civilians a day earlier.

After Turkey’s Parliament voted to authorize further military action against Syria, Prime Minister Recep Tayyip Erdogan said that another Syrian shell had landed in Turkish territory, the newspaper Today’s Zaman reported. Despite assuring reporters that his government “could never be interested in something like starting a war,” as he stood next to Iran’s visiting vice president, Mr. Erdogan added: “The Turkish Republic is a state capable of defending its citizens and borders. Nobody should try and test our determination on this subject.”

Late Thursday, thousands of antiwar protesters marched to Taksim Square in the center of Istanbul. The blogger Serhatcan Yurdam documented the protest using Instagram, and curated a collection of photographs uploaded to Twitter by observers and participants in the march.

RT @TWITERGAZETESI: İstanbul – Taksim / #savaşahayır mitingi onbinlerce insan yürüyor. //t.co/pMDZgEuq — Paul Ragsdale (@DOA3addict) 4 Oct 12

Raw footage of the protest posted online by the Turkish news agency DHA showed the protesters chanting “No to War!” and “U.S.A. Get Out of the Middle East!”

Among the slogans written on signs and banners carried by the protesters were: “This War Is Not My War,” “No to Imperialist Intervention in Syria,” “We Will Halt the AK Party’s War Politics,” “We Won’t Be the Soldiers of Imperialism,” and “U.S.A. Take Your Hands Out of the Middle East.”

An overhead view of the protest, posted on a Turkish-language Facebook page for supporters of President Bashar al-Assad of Syria, gave a sense of the scale of the demonstration.

The sound of predawn shelling from Turkey into Syria early Thursday was caught on video by the Turkish news agency Cihan.

The Turkish military action followed the death of five civilians on Wednesday when mortar fire from Syria killed two women and three children in the southern Turkish border town of Akcakale. Video of the chaotic scene moments after the shelling was broadcast on Turkish television.

At funerals for the five victims on Thursday, the newspaper Hurriyet reported that “Akçakale residents expressed their fury at the authorities who came.” Addressing civilian and military officials, one mourner asked, “You are protected by so many men, why are we unprotected?”

Juliette Cezzar contributed reporting.