The 24-year-old player, who had returned to Venezuela after his rookie season, was just outside the front door at his home Wednesday night when an SUV approached, armed men got out "and they took him away," said Ramos' agent, Gustavo Marcano.
It was the first known kidnapping of a Major League Baseball player in Venezuela, though the relatives of some ballplayers have previously been held captive for ransom.
Police found the kidnappers' vehicle abandoned in the nearby town of Bejuma on Thursday morning, Justice Minister Tareck El Aissami said. He said anti-kidnapping units led by "the best investigators we have" were dispatched to the area in central Carabobo state.
He vowed to rescue Ramos and capture his abductors.
"We're taking on this investigation with everything we've got," El Aissami said.
Major League Baseball and the Nationals said the leagues' Department of Investigations was working with authorities.
"Our foremost concern is with Wilson Ramos and his family and our thoughts are with them at this time," the team and the MLB said in a joint statement, adding there would be no further comment.
Ramos was outside with his father and two brothers in their working-class neighborhood on the outskirts of Valencia when the SUV pulled up with four men inside, three of whom got out and seized the player, Marcano said.
"The abductors haven't made contact with the family or with anyone," said Domingo Alvarez, vice president of the Venezuelan Professional Baseball League. "We're worried."
At baseball games in three Venezuelan stadiums, fans and players observed a minute of silence Thursay night in support of Ramos, and prayed for him. Some held signs reading: "Free Wilson Ramos!"
Julio Franco, a Dominican former major leaguer who is manager of the Venezuelan Caribes team, grabbed a microphone and spoke to fans at a game in the eastern city of Puerto La Cruz. "Jesus, we ask you for your grace and divine power to protect our brother, Wilson Ramos," he said.
To show support, teams added green to their uniforms, some sporting a green band on one shoulder, others a green "W" for Wilson embroidered on the chests of their jerseys.
Ramos is a key young player for the Nationals. As a rookie in 2011, he hit .267 with 15 home runs and 52 RBIs in 113 games. He also threw out 19 of 67 runners attempting to steal a base, a 28 percent success rate that ranked third among qualifying catchers in the National League.
Washington acquired Ramos from the Minnesota Twins in a trade for All-Star relief pitcher Matt Capps in July 2010.
He is one of dozens of Venezuelans in professional baseball, and security while at home has increasingly become a concern for the players and their families as a rising wave of kidnappings has hit the wealthy as well as the middle class.
Venezuelan police said 618 kidnappings were reported in 2009, and the numbers have grown rapidly in recent years. In 1998, when President Hugo Chavez was elected, just 52 kidnappings were reported. Security experts say the real number of kidnappings today is much higher because many cases aren't reported to authorities.
The wealthy have taken steps to protect themselves. Sales of armored cars have soared in the past several years. Bodyguards typically shadow major leaguers when they return to their homeland to play in the winter league.
"Every major league player has his own security, but we don't know if he had his security there at that time," Alvarez said.
Katherine Vilera, spokeswoman for Ramos' Venezuelan team, the Aragua Tigers, said on Twitter that neighbors gathered at his home and prayed.
Former Boston Red Sox slugger Tony Armas, who lives in Venezuela, said young players have been taking additional security measures due to the risk of kidnappings.
"But many of them are careless sometimes. No one seriously thinks that this can happen to us, and much less in a country like ours where people love baseball," Armas said in a telephone interview.
"Most of us came from humble families. We still have relatives who live in poor areas; we frequent those places and unfortunately the criminals are getting more soulless all the time," he said.
In November 2009, the 56-year-old mother of Victor Zambrano, who retired after a seven-year Major League career, was rescued in a commando-style operation three days after she was kidnapped. The former pitcher's cousin, Richard Mendez Zambrano, had been kidnapped a few days earlier, and was later killed.
In June 2009, Colorado Rockies catcher Yorvit Torrealba's 11-year-old son and brother-in-law were kidnapped and released a day later.
The mother of former player Ugueth Urbina, who was a two-time All-Star pitcher, spent more than five months in captivity until she was rescued in early 2005.
Venezuela has one of Latin America's highest murder rates, and violent crime has worsened in recent years. As ransom kidnapping has soared, the government passed a revised law in 2009 that stiffened prison sentences for kidnapping and also allows authorities to freeze the banks accounts of victims' families to prevent them from paying ransom.
Ramos had been training and planned to start playing with his Venezuelan team next week. Arizona Diamondbacks third baseman Melvin Mora, also a Venezuelan, proposed that the Venezuelan league ought to call off its games "until he appears."
But league president Jose Grasso said that won't happen. "Turning out the stadium lights isn't a solution," Grasso said, calling Ramos' abduction "an isolated event."
AP Sports Writers Howard Fendrich in Washington and Ron Blum in New York and AP freelance writer Billy Russo in Caracas contributed to this report.