A San Francisco police officer who reported using force more often than any other officer in the department during a recent nine-year period is at the center of four incidents in the last nine months in which citizens say they were subjected to excessive force.

In one incident, a young man who told Officer Jesse Serna he would be making a complaint against him said that moments later he was thrown to the ground by officers in North Beach and was zapped 12 times with a stun gun as he lay handcuffed on the street.

In another North Beach incident, a man said Serna hit him twice with his baton after he told police they were using force on someone who had been the victim of an assault.

In a third case, on the Embarcadero, witnesses said Serna and a partner wrestled to the ground a man who had called out that he would be a witness for another man whom Serna had hit while he was arresting him. Serna said in his police report that he went on to pepper-spray the would-be witness' wife and another woman.

In these cases, citizens said Serna had exhibited racial prejudice.

In a fourth case, a woman alleged that Serna used a sexual slur against her after she had joked with him and other officers about fearing what they would do to her if she tried to cross the street.

Two of these cases have resulted in lawsuits, and a third has raised the possibility of one. Since he joined the department, Serna has been involved in three lawsuits alleging excessive force that cost taxpayers a total of $195,000.

Sixteen months ago, The Chronicle published a series of articles on the San Francisco Police Department's use of force. For that series, the newspaper created a database of officers' 1996-2004 force reports and identified Serna as by far the highest reported user of force in the 2,200-member department.

In that period, Serna reported using force 57 times and injuring 31 people. His tally of force-involved incidents was 50 percent higher than any other officer.

Nine times in that same period, Serna made the watch list the department maintains to identify frequent users of force, according to documents provided by the department.

Since the use-of-force articles were published, The Chronicle has requested copies of the department's watch lists for the years 2005 and 2006. The department declined to provide the watch lists, maintaining that they were personnel records protected from public inspection by state law.

The latest incidents place renewed focus on Serna, who joined the force in 1995 and is assigned to Central Station.

He is the stepson of one of the department's highest-ranking officers -- Police Commander Stephen Tacchini. Tacchini, who supervises the city's 10 district stations, including Central, did not return The Chronicle's calls.

Police Chief Heather Fong said a commander in Tacchini's position would not be involved in investigations of officers at the district station level.

Fong said privacy provisions in state law bar her from commenting on personnel matters involving any officer.

She said she did not learn about "the matter of these four cases" until a call from The Chronicle on Tuesday afternoon requesting comment.

She added:

"I have directed the captain of the station to review these cases and make a recommendation regarding what he believes would be appropriate action."

Efforts to reach Serna, 41, for comment were unsuccessful. The department said he was on vacation.

Steve Johnson, business agent for the San Francisco Police Officers Association, said, "I personally know Serna. I worked with him at Mission Station, and he's an outstanding officer."

What follows is a detailed account of the incidents involving Serna based on interviews, court filings and other documents:

Greg Oliver

At 1:45 a.m. in North Beach on Aug. 20, 2006, Barry Bonds' personal trainer, Greg Oliver, left the Impala Club at the corner of Broadway and Kearny, where he had been with several friends.

As he walked along Broadway looking for a taxi, he saw three men attack another man. Police quickly responded, and Oliver said that he tried to tell the officers that they were using force on the man who had been the victim of the assault.

When an officer who he later learned was Serna ordered him to back up, Oliver said he complied. But then, he said, Serna hit him twice with his baton. When he then asked for Serna's badge number, he said, Serna grabbed Oliver by the neck and shirt, breaking the necklace he was wearing and telling him he was under arrest.

At that point, Oliver said he yelled out he was not resisting but was thrown to the ground by several other unknown officers who began punching him and kneeing him in the back.

When Serna pulled Oliver up from the ground, Oliver said, Serna wrenched his left wrist so hard that he screamed. In response, Oliver said Serna told him to "shut the f -- up" and continued wrenching his wrist.

According to a lawsuit filed in federal court later by his attorney, John Burris, Oliver was arrested on "fabricated charges that he resisted, obstructed or delayed a peace officer in performance of their duties," but no charges were ever filed against him.

Alleging that Oliver's civil rights were violated, the lawsuit seeks $1 million in damages. The suit also notes that Oliver is "readily recognizable as African American" and says that Serna and the other officers were "motivated by racial prejudice."

Burris said that the fact that Serna still works the streets reflects "an attitude that the department undoubtedly has where it would rather have this tough, abusive guy on the force than control him. It sends the totally wrong message to officers who don't engage in this kind of conduct."

Mehrdad Alemozaffar

At about 2 a.m. Dec. 17, 2006, Mehrdad Alemozaffar, a 26-year-old UCLA medical student visiting the city, was waiting on the street for a friend to buy some pizza from a shop on Broadway that was about to close.

Just then, Alemozaffar said, three police officers came along, yelling for everyone to get off the street. Alemozaffar said he and two young women who were walking next to him were pushed in the back by the police.

"They were pushing us for no reason -- I said, 'Stop pushing us, we're walking' and they were like, 'walk faster,' " Alemozaffar recalled in an interview. He said he ended up standing behind a metal police street barrier near Broadway and Montgomery and told the officers, "I am just waiting for a friend. She's my ride."

At that point, he said one of the officers -- he later learned it was Serna -- said to him: "Stop acting like such a girl." Alemozaffar, who is of Persian descent, said he was "taken aback -- I am in San Francisco, a liberal town. I said to him: 'What? Why would you call me that? I am obviously not a girl. Are you discriminating against me or what?' "

"He said in reply, 'I am calling you a girl because you are acting like one,' and he laughed. I will never forget that laugh -- it was like, 'I have more power than you and can say whatever I want.' "

Alemozaffar said he looked at Serna's badge and said, "OK, Officer Serna, I will make sure I take this up with the right people."

He said that as he and the young women began to walk away, he was grabbed from behind by officers -- he never knew how many -- and was dropped to the ground. He said, "I didn't put up a fight" as police put his hands in plastic handcuffs and smashed his face "repeatedly into the ground."

"I am screaming 'Stop, I am not fighting you,' and the next thing I feel is this jolt up and down my body. I was already face down on the ground and in handcuffs before I was tasered. I passed out a couple of times. I remember one jolt that lifted me off the ground and my entire back was arching -- it was unbelievable pain."

A sheriff's deputy, J. Reymunudo, was using a stun gun, the department said. His report, supported by a partner's, says he was assisting Serna and other police officers who were having trouble taking Alemozaffar into custody and that he used the device twice on Alemozaffar until he was subdued.

The Chronicle obtained photographs of Alemozaffar taken after the incident. They show many burn marks that he said were made by the stun gun.

Sheriff Michael Hennessey, asked about Alemozaffar's assertion he was hit by a stun gun 12 times while handcuffed, said: "I can assure you no deputy sheriff would use a stun gun on a handcuffed prisoner, or they would be fired, and I think all my deputies know that."

Alemozaffar said police cited him for resisting arrest and battery on a police officer.

Since then, he said, he has found three people who witnessed what happened to him. One gave him a murky video of the incident taken with a camera phone, which was reviewed by The Chronicle. The voices of people watching the incident can be heard.

"Kneed him and tasered him," a voice says. "They are still going at him!" one says. "They are still tasering the s -- out of him!" and "He didn't even swing on him, dude" and "No, he didn't do nothing."

Alemozaffar, who says he will begin a medical residency at Harvard University in the fall, blames what happened to him on "wrong place, wrong time, wrong color."

"I look brown," he said. "Deep in my heart I think what happened is you have a bad cop who thinks he can get away with doing and saying whatever he wants, and he thought I was some punk kid. This guy does not belong in a position of power."

Since Alemozaffar's encounter with Serna, he has hired two lawyers. The lawyer in his criminal case, John Runfola, said the district attorney decided there was not enough evidence to file charges against his client. Alemozaffar's other lawyer, Sanford Cipinko, said he will file a claim against the officers and deputy involved as well as the police and sheriff's departments, and he plans to file a lawsuit if a settlement is not reached.

Alemozaffar said he wants to see "Serna off the street. I feared for my life that night. I felt like I was going to die after being tasered repeatedly. I had a right to ask for his badge, but Serna has a personal agenda of taking the law into his own hands. He escalated a nonviolent situation into a potentially life-threatening one."

Shawn and Sarah Myers

Late in the afternoon of Feb. 24, 2007, at a spot near the Ferry Building on the Embarcadero, Serna and a partner were arresting a man who had been involved in a fight with a motorist.

In his police report, Serna said he got a handcuff onto the left hand of Jamal Jackson, 18, of San Francisco, but the suspect pulled his right hand away.

"Fearing that he may have been reaching for a weapon, I struck Jackson with my right fist on his lower back because I had no other force option available to me to detain the suspect," Serna wrote.

Jackson then called out, asking if anyone had seen what Serna did. Shawn Myers, 34, an African American San Francisco resident who was waiting with his wife for his car at a nearby parking lot, responded that he had seen what happened.

Myers' wife, Sarah, 32, wrote in a statement that day that Serna immediately came over and told her husband to put his hands behind his back. As her husband was being forced to the ground, Sarah Myers said, "I stood there asking why were they doing this," and at that point "I was pepper-sprayed in the face by Officer Serna."

In his report, Serna contended that Shawn Myers had approached him in an aggressive posture with his hands in his jacket pocket, telling Serna, "F -- you, shoot me, mother -- ."

Serna reported he had to pepper spray Sarah Myers after she had "run towards us with her arms out as if she were going to push us or possibly attempt to free Myers." He said he had to pepper-spray another woman who also rushed at him.

Jackson, the man who Serna was trying to arrest, wrote a statement later identifying the second woman as his girlfriend.

Jackson said that after his girlfriend was pepper-sprayed, he fled the scene to avoid being pepper-sprayed himself. He said he was caught and placed in the back seat of a patrol car. He added that Serna jumped in the back seat with him and "started hittin' me on my side back of my head because I had my head down; he pulled my head up and hit me again, callin' me 'n -- .' "

Five witnesses wrote statements, included in the police report, that were critical of the police conduct in the incident.

Myers' San Rafael lawyer, Matt Mani, denied that Myers had sworn at Serna. "All my client did was see an injustice and offer to be a witness," Mani said.

He added that after Myers was cuffed and in a police van, Serna repeatedly used racially charged language to disparage both Myers and his wife.

Reflecting on the encounter with Serna, Sarah Myers told The Chronicle, "Prior to this incident, I've never had a negative encounter with the police. I have a number of friends who are officers, and I have a great deal of respect for them."

Myers, who faces an Aug. 1 court hearing on charges of interfering with an officer, battery on an officer and resisting arrest, said that Serna at one point called him "boy" and after placing him in a police van, called him "monkey" and said "monkeys belong in cages." Myers also quoted Serna as saying: "That sure is an ugly white bitch you're married to."

Esther Hwang

On May 12, 2007, at about 10:30 p.m., Esther Hwang -- a pin-up model and a former personal scheduler for Mayor Willie Brown -- was waiting for her boyfriend outside the Dolce club in North Beach when she saw some officers nearby.

Hwang, who said she had been celebrating after finishing exams at San Francisco Law School, said that, as a joke, she called out flirtatiously, "I guess you'll have to tackle me if I were to try to cross the street."

Suddenly, she said, one officer -- who she later learned was Serna -- twisted her arm behind her back while another grabbed her from behind. She said Serna yanked her to the ground by her hair and said to her, "You f -- c -- ."

Those words, she told a crowded news conference Wednesday, made her "feel small and powerless."

Put into a police wagon, she was driven to Central Station where she said she passed out due to an anxiety attack, hurting her head in the process. Later she was cited for battery on an officer and resisting arrest and was released.

Her lawyer, John Burris, told reporters that he had filed a federal civil rights lawsuit against Serna and the department seeking $1 million in damages.

Burris said the police reports say she was "belligerently drunk," but he said Hwang had only consumed one drink.

Hwang is due in Superior Court late next month to face charges of resisting arrest and battery on a police officer. "What got to me the most," Hwang said, "is Officer Serna does this to people on a random basis, and no one does anything about it."