The leader of the assasination was Count Peter Alexeyevich Pahlen. He was the Governor of St. Petersburg and was trusted by Paul. Pahlen was supported by General Bennigsen, his friend, the Zubov brothers (one was Catherine the Great's last lover) and a number of commanders as well. They knew they needed to gain support from Alexander but he was indecisive and he rejected an earlier conspiracy led by Panin (which failed of course).
However, Paul became more crazy and threatened to replace Alexander with Eugene of Wurttemburg as Czarevich. Paul also made Alexander read the passage descriping the death of Czarevich Alexei, who was tortured to death by Peter the Great.
Pahlen emphasized that they did not want to take the life of Paul and they only wanted Paul to abdicate in favor of Alexander. Once he had abdicated, he would be send away to retire with his wife and/or mistress. Alexander finally agreed because he thought that wouldn't splatter blood onto his hands.
The night before the assasination, Paul was rude, perhaps he guessed the assasins were coming soon. After a concert, he looked at his wife "up and down with a sneer, crossed his arms on his chest, and nosily blew out his breath with his nostrils distended, his pupils small and hard". Then he gave Alexander and Constantin the same look. At last, he berated Pahlen . At dinner, his family tried to thank him accofding to Russian custom but he gave them a sacrastic smile and left the room without saying good night to anyone.
On the night of March 11, the 3rd battalion of the Semeonovsky regiment, which Alexander commanded, was in charge of guarding the castle. That night, Paul was more friendly during dinner but he found every mirror in the dining room had a defect. He said to General Kutuzov, "It shows me with my neck crooked."
At 11 o'clock, about 50 conspirators made their way to the castle in 2 groups, one lead by Pahlen and the other by Plato Zubov and Bennigsen. The entered the castle through a side drawbridge, "climbed a narrow spiral staircase, and slipped into a library that served as antechamber to Paul's apartment". The door was guarded by two sleepy footmen. Awakened by the noise, one collapsed and the other fled in terror.
Approx 10 men followed the Zubovs and Bennigsen into the bedroom but found it empty. Bennigsen spotted bare feet behind a screen. They grabbed the Emperor and forced him to abdicate. Paul refused and Bennigsen and Plato Zubov left the room. Paul continued to struggle against the officers and during the scuffle, the candle went out. Somebody threw a gold snuffbox and struck Paul on the temple. He collapsed and out of fear, some officers stranggle Paul to keep him quiet. Paul, struggling for air thought he saw his son Constantine and pleaded for mercy. Finally Paul died. His wife heard the screams and rushed to his room screaming murder in German. Bennigsen's soldiers stopped her while the others tidied the room up.
From Alexander of Russia by Henri Troyat