It’s been a whirl of activity in the international art market. Following the highly successful New York auctions last month, members of the art world toured through Europe for the “Grand Tour”: the Venice Biennale in Italy, Art Basel in Switzerland, Documenta 12 in Kassel, Germany, and Sculpture Projects Münster, also in Germany. And they were buying all along the way.

This week, the crowd arrives in London for the June auctions of Impressionist, Modern, and Contemporary art. Unlike the two-week orgy in New York, London’s sales come fast and furious within one week. Today and tomorrow, Impressionist and Modern masters will get hammered down at Christie’s and Sotheby’s.

Claude Monet was the star of New York a month ago. All but one of the five paintings in the prestigious evening sales sold at prices well above the high estimate. If New Yorkers are aggressive for the Impressionist master, Londoners are gaga. Here is where the record price for Monet was set in 1998 with a painting going for just under $40 million. No wonder then, that three of the top four lots on offer in London this week are by Monet. Sotheby’s has the big-daddy painting: “Nymphéas” (1904), estimated between $20 million and $30 million at the high end.

Christie’s isn’t far behind with formidable paintings. The first is a flower painting, “Les arceaux de roses, Giverny” (1913), which comes estimated at $18-24 million. Monet’s garden is still a mythic touchstone for his art and this work has been with the same owner since 1965 while similar paintings hang in museums.

That doesn’t necessarily mean this one will wind up in a museum. New collectors are changing every aspect of the art market. “We definitely have the impression that a lot of new buyers are entering at the top end,” Sotheby’s expert Helena Newman said. “People who’ve made a lot of money very fast are not collecting modestly,” she said of the new buyers’ desire to own the best works by iconic artists.

One of the iconic Monet images is the Waterloo Bridge viewed from London’s Savoy hotel. Matisse stayed there a number of times between 1899 and 1901 and painted the view of the bridge 40 times. In fact, he painted it many, many more times, but it took several years before the artist could complete 40 of the canvasses.

“This is one of the two best,” Christie’s Impressionist expert Olivier Camu said of the Waterloo Bridge paintings, the majority of which are in museums. Less than a dozen are in private hands: This one comes from a Japanese collector. “Waterloo Bridge, temps couvert” (1904) is estimated between $12 million and $24 million.

Monet may have been a big hit in New York, but some other important artists hit a wall. Matisse and Modigliani both had significant paintings that failed to sell. That hasn’t scared off the Brits. Sotheby’s is high on Henri Matisse’s “Danseuse dans le fauteuil, sol et daumier” (1942), estimated at $16-24 million. “I think it’s a classic Matisse,” Sotheby’s Ms. Newman said.

The record for a Matisse is $18 million. So this painting should set a new record for a giant of the 20th century. The painting, with a strong black-and-white checkerboard pattern and bright reds and yellows, has great visual appeal and wall power. Anyone who buys it will be making a statement. And it seems unlikely that Matisse will stay undervalued much longer.

On the flip side, Modigliani had a rapid run-up in price that stopped in New York. Sotheby’s has “Jeune femme” (1917) estimated between $7 million and $9 million This price level is much more realistic and this picture has other advantages over the paintings in New York: It’s been with the current owner since 1982.

London has a final advantage in these days. Europeans are experiencing a wealth surge and the weak dollar has leveled the playing field further for new collectors from the former Soviet states, Europe, and the Middle East. “The Americans are still there,” Christie’s Mr. Camu said. “They tend to be outbid, but they are supporting the market.”