If you go back to Attack of the Clones, Anakin and Obi-Wan sensed that poisonous alien worms were about to kill Padme. Back in Revenge of the Sith, the Jedi could take on the legion or if there were too many, they could at least escape.
The Jedi at the temple were at a massive disadvantage. The most capable Jedi were offworld fighting leading the Clone Army and those that were left on Coruscant were the young, the elderly, a few guards, various Temple staff (librarians, mechnanics, slicers and such) and a few members of the Council who weren't in the building when the attack happened.
Once Order 66 was enacted, the Jedi were systematically gunned down across the galaxy over the course of just a few minutes. Those that remained alive seem to be suffering from some sort of Force-related shock at the change in the balance of the Force from light to dark. On top of that, the Clones aren't actually malevolent, merely following their orders dispassionately:
With the Jedi Order overextended, spread thin across the galaxy, each Jedi is alone, surrounded only by whatever clone troops he, she, or it commands. War itself pours darkness into the Force, deepening the cloud that limits Jedi perception. And the clones have no malice, no hatred, not the slightest ill intent that might give warning. They are only following orders.
In addition to what Valorum said, I would speculate that the idea that the dark side was "clouding" Jedi ability to sense various dangers (especially those linked to the dark side, like those of the Sith or fallen Jedi) serves as a catch-all explanation for why the Jedi didn't anticipate these dangers. From the "Behind the Scenes" section of the wookieepedia "Chosen One" article:
Several stories suggested that the Sith were unbalancing the Force by strengthening the dark side and weakening the light side for Force-users everywhere. In Attack of the Clones, Mace Windu said to Yoda that "I think it is time we informed the Senate that our ability to use the Force has diminished." Also in that film, Dooku told Obi-Wan the Republic was under the control of a Sith, and Obi-Wan replied "No, that's not possible. The Jedi would be aware of it", to which Dooku replied "the dark side of the Force has clouded their vision, my friend." In the Star Wars: The Clone Wars episode "The Wrong Jedi", Ahsoka Tano had been accused of a crime and said she couldn't tell who had really done it because her senses were "clouded", and Yoda responded "Clouded by the dark side these things are, Padawan Tano. Dangerously clouded. But not just surrounding you. Surrounding many things, in these times." The Revenge of the Sith novelization referred to "the darkening cloud upon the Force that had risen with the rebirth of the Sith" which made it harder for Mace to see the significance of structures of people and situations he perceived in the Force, and similarly the novel Shatterpoint referred to "the shadow that had darkened Jedi perception of the Force".