Match ends, Watford 3, Liverpool 3.
Miguel Britos scored a controversial 93rd-minute equaliser for Watford as Liverpool were held in a thrilling game at Vicarage Road.
The Reds, without Barcelona target Philippe Coutinho due to a back injury, fell behind to Stefano Okaka's header.
Sadio Mane equalised with a clinical finish only for Abdoulaye Doucoure to restore the lead from close range.
Roberto Firmino's penalty made it 2-2 before Mohamed Salah thought he had won it only for Britos to bundle in.
In Jurgen Klopp's 100th competitive game as Reds boss, Liverpool wasted a string of chances before Uruguay defender Britos headed over the line from point blank range following a ricochet.
Watford keeper Heurelho Gomes, who conceded the penalty after hauling down Salah, denied Alberto Moreno with a fingertip save.
Joel Matip also hit the bar and Dejan Lovren forced another good save as the Reds dominated the closing stages until Watford's late leveller.
The six-goal thriller came on the back of Friday's opening match of the 2017-18 campaign which produced seven goals as Arsenal beat Leicester 4-3.
Familiar frailties on show for Liverpool
It was a case of new season but same old Liverpool on the opening Saturday of the campaign.
As good as Liverpool were at going forward, their defensive frailties - all too evident last season - came back to haunt them.
Southampton defender Virgil van Dijk has been linked with a move to Anfield and Liverpool's performance at Watford will only serve to increase Klopp's urgency to strengthen his defence.
Liverpool were behind after failing to defend the first corner of the match, unmarked Italian striker Okaka allowed to power home Jose Holebas' delivery.
In the space of 161 first-half seconds, the Reds went from outstanding at one end of the pitch to atrocious at the other.
They equalised with a sumptuous finish by Mane, the Senegal forward collecting a lovely flick by Emre Can before scoring from an angle.
It was Liverpool's first serious attack of the game yet moments later they were trailing again after more awful defending.
The Reds' defence made a hash of trying to clear Tom Cleverley's low ball across the box and Doucoure slotted home from five yards - his second goal since joining Watford from Rennes in 2016.
A clumsy foul by Gomes on Salah allowed Firmino to make it 2-2 from the spot before the Brazilian turned provider with a delicious lob which enabled Salah, a £34m signing from Roma, to put his side ahead on his league debut.
It should have been the winner yet Liverpool's defence failed to deal with another Holebas corner, allowing Britos to score three minutes into added time.
'Watford's equaliser was offside'
Klopp said Watford's dramatic equaliser should not have counted because he believed it to be offside.
Britos was stood immediately in front of keeper Simon Mignolet when substitute and new signing Richarlison put the ball across goal.
"It's obvious because the linesman is on the line. He needs to see it," said Klopp.
"I cannot change it. We would have had defensive areas we need to work on if we'd won 3-2. We were the better team, we should have won."
Watford boss Marco Silva said he had not seen whether his side's equaliser was offside.
Battling Watford show spirit under Silva
Silva's side were minutes away from losing a seventh successive Premier League match before Britos had the final say in the former Hull City manager's first league game in charge of the Hornets.
Seven of the side that started against Liverpool were part of the team that was crushed 5-0 at home to Manchester City on the final day of last season.
Yet the Hornets produced an encouraging performance under their new Portuguese boss which suggests they could finish higher up the table than last season's 17th-placed finish.
Midfielder Nathaniel Chalobah was the only one of Silva's six summer signings to start - club record £18.5m buy Andre Gray beginning life on the bench and coming on in the second half.
Man of the Match - Roberto Firmino (Liverpool)
Watford manager Marco Silva said: "I think it's a fair result. We had a very good first half, a good intensity like I wanted.
"I didn't like the first 25 minutes of the second half. We expected a good reaction from Liverpool but we needed to be better. We made mistakes. But we reacted well.
"Our fans pushed our players on. We had some small injuries. We need to show this attitude and commitment in all our games."
Liverpool manager Jurgen Klopp said: "The first half I wasn't too happy with. We passed but couldn't see what we actually wanted.
"The second half was a lot better. We forgot to close the game. We defended a little deep. We have to push up. Both teams struggled with 90 minutes, it's the first game. That's normal."
Defender and deadly goal provider... the stats
- No defender has provided more assists in the Premier League since the start of last season than Jose Holebas (five).
- Liverpool's Sadio Mane has netted five goals in his past seven Premier League appearances.
- Mane has scored with his first shot on target in the Premier League in both the 2016-17 and 2017-18 seasons.
- Firmino has both scored more and assisted more Premier League goals than any other Liverpool player since the start of 2015-16 (22 goals, 15 assists).
- Salah is the 12th different player to score on his Premier League debut for Liverpool.
- Only Jose Fonte (five) has given away more penalties in the Premier League than Heurelho Gomes (four) since the start of 2015-16.
Liverpool will hope for a better defensive performance when they face Hoffenheim in Germany on Tuesday in a Champions League play-off (19:45 BST). Watford have a week to prepare for an away game at Bournemouth on 19 August (15:00 BST).
- 2JanmaatSubstituted forFemeníaat 18'minutes
- 7N Amrabat
- 33OkakaSubstituted forGrayat 63'minutes
- 37PereyraSubstituted forRicharlisonat 49'minutes
- 22MignoletBooked at 88mins
- 66Alexander-ArnoldBooked at 61minsSubstituted forGomezat 90+1'minutes
- 11SalahSubstituted forMilnerat 86'minutes
- 9FirminoSubstituted forOrigiat 81'minutes
- 19ManéBooked at 37mins
- Anthony Taylor
- Shots on Target
Second Half ends, Watford 3, Liverpool 3.
Attempt blocked. Georginio Wijnaldum (Liverpool) right footed shot from the centre of the box is blocked. Assisted by Joseph Gomez with a cross.
Delay over. They are ready to continue.
Delay in match Richarlison (Watford) because of an injury.
Attempt missed. Richarlison (Watford) header from the centre of the box misses to the right. Assisted by Nordin Amrabat with a cross.
Goal! Watford 3, Liverpool 3. Miguel Britos (Watford) header from very close range to the centre of the goal following a corner.
Attempt missed. Richarlison (Watford) right footed shot from the right side of the six yard box misses to the left following a corner.
Corner, Watford. Conceded by Simon Mignolet.
Attempt saved. Miguel Britos (Watford) left footed shot from outside the box is saved in the top centre of the goal. Assisted by Abdoulaye Doucouré with a headed pass.
Foul by Joseph Gomez (Liverpool).
Richarlison (Watford) wins a free kick on the left wing.
Substitution, Liverpool. Joseph Gomez replaces Trent Alexander-Arnold.
Simon Mignolet (Liverpool) is shown the yellow card.
Substitution, Liverpool. James Milner replaces Mohamed Salah.
Attempt missed. Georginio Wijnaldum (Liverpool) right footed shot from the centre of the box misses to the right. Assisted by Sadio Mané.
Substitution, Liverpool. Divock Origi replaces Roberto Firmino.
Delay over. They are ready to continue.
Delay in match Younes Kaboul (Watford) because of an injury.
Attempt missed. Mohamed Salah (Liverpool) left footed shot from the right side of the box is close, but misses to the right. Assisted by Sadio Mané.