Ed Sheeran revealed that he had to drastically alter his stage performance for his Philadelphia concert ... and it was all because he was worried fans might pelt him mid-show!

At one point in the performance ... he told the crowd that he initially was going to perform on a B Stage ... but he decided against it.

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He said ... "I didn't know what this audience would be like. I didn't know if I would have things thrown at me."

Explaining why he took the B Stage out, he added ... "I was afraid of what might happen if I stood in the middle. And I will admit that."

We reported earlier ... Ed took the stage last night and publicly commented on the last week that saw every opening act of his quit the tour after Macklemore was axed for his pro-Palestine speech.

While addressing the crowd at the Lincoln Financial Field ... Ed told his fans he was "so, so sorry."

He said ... “I don’t want to disappoint fans, and I’ve never wanted to be an activist musician, but this has put me in the middle of an important and passionate argument about freedom of speech and the most complex political issue on the planet."

Security was obviously a big concern last night ... but it seemed to go off without a hitch.

Philly PD previously told TMZ that they'd be taking "appropriate measures" for everyone's safety.