By the hour mark, the Argentine captain had made it 3-0.
Late goals from Gerard Pique and Luis Suarez completed the thrashing, with Dembele grabbing an assist, laying on the pass for the Uruguayan's goal.
After a slow opening, Barcelona took the lead after 26 minutes.
Suarez who shrugged off an injury whilst on worldwide break missed some decent chances, the closest being in the 55th minute when he blazed wide in the six yard box after a cutback from Messi.
'We are at the beginning of the season and the distances are not significant, but it is better to be ahead than behind.
However, the 22-year-old custodian could not repeat similar heroics during Barca's next effort on goal, which was fortunate to say the least, as Messi was on hand to simply tuck away his second of the evening following Jordi Alba's squared ball.
Far from fearing an imminent humiliation, Espanyol gamely responded and Barca goalkeeper Marc-Andre ter Stegen was grasping at air when Pablo Piatti's strike cannoned back off the post two minutes before half-time. "Game-by-game he keeps showing that Messi will be around for a long time to come".
Uruguay forward Suarez, who only returned from global duty on Friday, looked to be having one of those nights as he sent a cut-back from Messi over from six yards.
Five-time Ballon d'Or victor was in fine form, opening the scoring in the 26th minute, following up again ten minutes later, utilising expert service from Jordi Alba, sealing his hat-trick with more than 20 minutes remaining.
Espanyol were being outplayed but retained a sporadic threat, with Pique and Samuel Umtiti diverting Leo Batistao's 61st-minute shot behind.
The club promised two signings in the last week of the transfer window and none arrived and Bartomeu was booed by fans at Dembele's presentation in the Camp Nou stadium.
The French winger produced a quick show of footwork to turn Aaron Caricol, only to be brought down by the left-back, who was swiftly shown a yellow card.
Dembele has the unenviable task of replacing the departed Neymar and Barca's 105 million-euro man had to wait until 22 minutes from time for his debut as he replaced Gerard Deulofeu to a standing ovation.
Earlier in the day, seven coaches of Howrah-Jabalpur Shaktipunj Express skid off the railway track near Sonbhadra, Uttar Pradesh. On August 25, four coaches of local train in Mumbai had also derailed due to a technical fault injuring six passengers.
Dilshan Munaweera struck 53 and Ashan Priyanjan 40 not out in vain as the hosts finished a wretched run of clashes against India . Pandey finished off the game with a boundary that brought his maiden T20 fifty in 36 balls. "I want to play all the games".
In its statement, the bank also said headline and core inflation have seen slight increases since July, largely as expected. At its previous meeting in July, the BoC raised its key rate for the first time in seven years, also by 25 basis points.
He remembers past hurricanes and tropical storms, but never he says has he heard fear in his 80-year-old father's voice until now. But after the costly affects of Harvey, he's already working with Congress on additional funding for the island.
Defense Secretary Jim Mattis on Sunday said the USA will answer any threat from the North with a "massive military response". President Donald Trump has spoken with German Chancellor Angela Merkel to discuss the latest provocation by North Korea.
He added: "Physically I'm not in ideal condition, but when you play in the semi-finals of a Grand Slam , anything can happen. Del Potro, who beat Nadal in an epic Rio Olympic semifinal, knocked out Federer in the quarterfinals Wednesday.
Though CPI (M) had offered him a ticket to contest in the next assembly polls, he turned down the offer and later joined the BJP . Alphons added that though he had no great experience of the tourism sector, he would do his best to take it to new heights.
Fantasy Impact Although Janikowski was placed on IR Saturday, he won't need surgery to fix the herniated disc in his back. Tavecchio kicked in college at Cal and bounced around different teams after going undrafted in 2012.
Connectivity options include 4G LTE connectivity, Bluetooth 4.0, USB 2.0, GPS, and Wi-Fi 802.11 b/g/n options. Storage wise, the phone offers 16GB of internal storage which is expandable up to 128GB using a microSD card.
But Alastair Cook, who floored two first-slip catches in the Windies chase in Leeds, dropped a seemingly routine chance. Westley will not want to see his dismissal again after he was trapped lbw by Holder on eight.
USA calls for UNSC vote on North Korea sanctions
Meanwhile, the U.N.is considering fresh sanctions to counter North Korea's increasingly frequent and powerful missile launches. North Korea on September 3 detonated a hydrogen bomb that can be carried by an intercontinental ballistic missile (ICBM).
2 dead, damage in Mexico after Hurricane Katia strikes
Hurricane Katia , the sixth hurricane of the 2017 season, is expected to make landfall in Mexico early Saturday morning. A hurricane warning means that it's expected to hit about 12-24 hours, the NHC noted in its update.
Three Hurricanes at once: Irma, Jose and Katia
The depression is forecast to strengthen into a tropical storm within the next day or so, and it will receive the name Katia . So while Houston will be spared from another storm, eastern Mexico is at risk of localised flooding and potential mudslides.
Android One: What It Is and All That You Need to Know
The chipset that ensures phones run longer and cool as a cucumber has been a runaway hit in the under Rs 20,000 price segment. The Xiaomi Mi A1 was unveiled in New Delhi , India earlier this week, boasting a lot of improvements in specs and design .
Broncos bring back Brock Osweiler
With Osweiler leaving, it appears the Browns will also keep quarterbacks Cody Kessler and Kevin Hogan behind Kizer. Osweiler , who sat out the Browns' final two preseason games, was healthy all throughout training camp.