Harry would be extremely frustrated. He has been through so much in his fight against Voldemort, and now this? He would feel betrayed by the very people he fought to protect. He would start by trying to reason with the authorities, but when that fails, he would resort to more drastic measures. He might try to escape custody to find the real evidence that could clear his name. He would also be worried about his friends and how they are being affected by this situation. His reaction would be a mix of anger, determination, and a bit of sadness at the turn of events.
At first, Harry would likely be in a state of disbelief. He has just defeated the most evil wizard in history and now he is being treated like a criminal. He might start to question everything he has done. However, as time passes, his Gryffindor courage would kick in. He would begin to gather evidence and allies to fight against this false accusation. He would probably also be a bit resentful towards the wizarding world for not seeing the truth. He would be thinking about all the sacrifices he made during the fight against Voldemort and how ungrateful the wizarding community seems to be.
One possible plot twist could be that Harry was set up. Maybe there was a powerful dark wizard who wanted to get rid of Harry and framed him for Voldemort's death in a way that made it seem like a cold - blooded murder rather than a heroic act. Another twist could be that the wizarding world has turned against Harry because they fear his power after defeating Voldemort and use this arrest as a way to control him.
Harry could also try to reason with Voldemort. He might remind Voldemort of the prophecies and how killing him might not be in Voldemort's best interest. He could also try to appeal to any bit of humanity that might still be left in Voldemort, although that's a long shot.
Harry would likely be defiant at first. He has a strong sense of justice and would not easily submit to Voldemort's punishments. For example, if Voldemort uses the Cruciatus Curse, Harry would probably try to resist the pain and not show weakness.
Well, in some fanfictions, Voldemort might see Harry as a tool to be used against his abusers. He could use this knowledge to manipulate Harry into joining his side, thinking that he can offer protection or revenge against those who have wronged Harry.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
The wizarding community could be in shock. Many would feel that their last hope is gone. Some might lose faith and start to submit to Voldemort's rule. Others, however, could be inspired to fight even harder, believing that they need to carry on Harry's legacy.
He would probably try to figure out what went wrong. Harry is a curious and determined character. He might start to recall recent events to see if there were any signs of his friends' discontent. He could also try to reach out to them, either directly or through mutual acquaintances, to ask for an explanation.
Over time, Harry might use this to his advantage. He realizes that Voldemort's perception of him as a baby could be a weakness in Voldemort's strategy. So he starts to act more meek and innocent in front of Voldemort, while secretly planning his next moves. This way, he can catch Voldemort off - guard when the time is right.
Some in the wizarding community might be horrified. His friends, like Hermione and Ron, would probably try to stop him at all costs. They would see it as a waste of Harry's life and believe that there must be another way to defeat Voldemort.
Harry might be extremely hurt. He could initially feel a sense of shock and disbelief. After all, he may have started to develop some feelings of trust or kinship towards Severus. Then, he might turn to anger. Anger at being rejected, especially if he doesn't understand the reasons behind it. He could also feel a sense of loss, as if he has been deprived of a chance at a real family connection.